For most large telecoms operators, the transformation journey does not begin with a blank slate. It begins with a live, complex, interdependent infrastructure that cannot simply be switched off. Engineers in the field, customers relying on services, and regulators monitoring continuity all demand that legacy systems keep running - even as new platforms are being introduced alongside them.

This is the coexistence problem: the challenge of operating old and new infrastructure in parallel, often indefinitely, without destabilising either. It is one of the most practically demanding situations in enterprise technology, and it is currently defining the operational reality for network operators across the UK and beyond.

Getting it right is not just a technical question. It is a strategic one.

1. The Scale of the Problem

Telecoms infrastructure has a longer operational lifespan than almost any other enterprise technology. Systems deployed in the 1980s and 1990s still carry live traffic today. In the UK alone, Openreach is managing one of the most ambitious infrastructure transitions in its history - migrating millions of customers from the legacy Public Switched Telephone Network (PSTN) to an all-IP, full-fibre environment, with a hard switch-off deadline of January 2027. That deadline, already delayed once from its original 2025 target, underscores just how complex and consequential this transition is.

The challenge is not confined to physical infrastructure. Across the industry, OSS (Operations Support Systems) and BSS (Business Support Systems) platforms that have been central to service delivery for decades now exist alongside newer cloud-native, API-driven replacements. According to research from EY, technology debt - the accumulation of legacy, siloed and suboptimal IT and network infrastructures - has become a material strategic and operational risk across the telecoms sector, hindering digital transformation and blocking the monetisation of 5G investments.

The OSS and BSS market is projected to grow from USD 65.81 billion in 2024 to USD 148.26 billion by 2033 - a compound annual growth rate of 9.4%. Much of that investment is driven by the urgency to replace or integrate systems that were never designed to coexist.

The financial and operational pressures compound each other. Legacy systems are expensive to maintain, difficult to find skilled resource for, and increasingly fragile. Yet decommissioning them prematurely - or trying to replace them in a single transformation programme - introduces its own risks: service disruption, data loss, and the failure to migrate customers who depend on older product types with no viable alternative yet available.

Also Read: Why TM Forum Compliance Is Becoming a Competitive Advantage

2. Why Coexistence Is Harder Than It Looks

Configuration Fragility at Scale

In a coexistence environment, a single misconfiguration - a misplaced character in a provisioning system, a mismatch between an old product record and a new platform's data schema - can take down a customer line. The interdependencies between legacy and modern systems are often poorly documented, having grown organically over decades rather than being designed. When something breaks, the diagnostic process itself is time-consuming because no single team has full visibility across both environments.

This is not a niche concern. It is the daily operational reality for field engineering and commissioning functions at major operators. It drives significant human intervention costs and means that even relatively minor changes to systems or configurations require extensive testing before deployment.

The Integration Gap

Legacy systems were designed to integrate with other legacy systems. Newer platforms are built for API-driven, microservices architectures. Connecting the two requires middleware, translation layers, and custom integration work - all of which introduces additional failure points and maintenance overhead.

TM Forum's Open Digital Architecture (ODA) initiative exists precisely to address this: creating a framework for composable, interoperable telecoms systems that can evolve without requiring a wholesale replacement of what already works. But adoption is gradual, and in the meantime, operators are managing point-to-point integrations that are difficult to change and harder to scale.

The Customer Migration Problem

Migrating customers from legacy to modern services sounds straightforward in theory. In practice, it involves coordinating across communications providers, managing customers who cannot or will not migrate voluntarily, and ensuring that edge cases - alarm systems on copper lines, healthcare telecare devices, payment terminals - do not fall through the gaps. In the UK, the government has published a Fixed Telecoms Modernisation Charter specifically to govern the safety obligations around this migration, reflecting just how consequential it is for vulnerable users.

Customers on legacy products often face significant cost barriers to upgrading - meaning legacy infrastructure must be maintained for commercial as well as operational reasons, long after it would otherwise be retired.

3. The Strategic Approaches That Work

Run AI and Modernisation in Parallel - Not in Sequence

A common misconception is that AI can only deliver value once legacy systems have been replaced. PwC's 2026 telecoms research challenges this directly, arguing that the same legacy complexity that blocks transformation is also where AI can help first. Well-scoped AI agents can take on targeted tasks - confirming orders, triaging network faults, flagging billing anomalies - while simultaneously surfacing the exact integration bottlenecks and data quality issues that need to be addressed next.

This approach means modernisation is evidence-led rather than ideology-led. Rather than committing to a multi-year programme based on assumptions about what will need to change, operators can use early AI deployments to identify where the real friction lies, and prioritise accordingly.

One major operator used AI-supported data classification and end-to-end lineage tracking to reduce data operations costs by approximately 50% - without waiting for legacy systems to be replaced. Teams spent less time diagnosing root causes and more time resolving issues that mattered.

The Strangler Fig Pattern for Systems Replacement

Rather than attempting a big-bang replacement of legacy systems, leading operators are using a modular, incremental approach - sometimes called the strangler fig pattern - where new capabilities are introduced alongside legacy systems and progressively take on more of the workload. Legacy systems are retired function by function, not all at once.

This requires a strong data streaming and event-driven integration layer so that legacy and modern systems can operate from the same underlying data without being directly dependent on each other. Apache Kafka-based architectures have emerged as a practical mechanism for this: allowing legacy systems to publish and consume data events without requiring them to be rewritten or replaced.

Investing in Observability and Unified Monitoring

In a coexistence environment, the greatest operational risk is the one you cannot see. Unified observability - the ability to monitor performance, data quality, and service continuity across both legacy and new platforms from a single vantage point - is a foundational investment. Without it, incidents in one environment propagate invisibly into the other before they can be detected and contained.

Digital twin technology is beginning to play a role here as well. By creating continuously updated models of network infrastructure that replace unreliable legacy records, operators can achieve a clearer picture of their actual asset state - making upgrades faster, budgets more predictable, and network expansions more efficient.

Addressing the Human Side of Coexistence

Technical approaches to coexistence will fail without the right operational and organisational model to support them. Coexistence environments require engineers and programme managers who understand both the legacy context and the direction of travel - people who can navigate between the two without losing sight of either. This is a scarcer skillset than it might appear, and the consolidation of telecoms workforces currently underway at several major operators is making it scarcer still.

Specialist resource - whether augmented from external partners or brought in as a managed capability - is increasingly how large operators are bridging this gap during transition periods, without taking on permanent headcount for functions that will diminish as modernisation progresses.

4. Where AI Is Having the Most Impact

Across the telecoms operators that are making measurable progress on legacy coexistence, AI is playing a consistent set of roles:

1. Fault prediction and triage - identifying likely failure points in legacy systems before they cause outages, and routing resolution tasks to the right engineers with the right context.

2. Configuration validation - detecting mismatches between legacy records and new platform expectations before changes are deployed, reducing the configuration errors that cause service outages.

3. Data quality and lineage - classifying, tagging, and tracking operational data across fragmented systems so that teams have a reliable picture of what they hold and where it comes from.

4. Automated order and provisioning support - handling routine service activations and order confirmations that currently require human intervention because legacy systems cannot communicate directly with modern fulfilment platforms.

5. Exchange consolidation planning - modelling the sequencing and dependencies involved in legacy site decommissioning, so that programmes can be executed efficiently without service disruption.

TM Forum's Autonomous Networks initiative - which aims to achieve large-scale adoption of self-managing, AI-native network operations by 2026 - represents the long-term destination. But the near-term value is in targeted, well-governed AI deployments that address the specific friction points of coexistence, not in waiting for the infrastructure to be clean enough for AI to work on.

5. The Commercial Case for Getting This Right

Legacy coexistence is often framed as a cost management problem. It is also a revenue opportunity. Operators that can manage the transition effectively will be faster to market with new services, more reliable in their delivery of existing ones, and better positioned to win commercial commitments from enterprise customers who need a credible modernisation partner.

Conversely, operators that allow the coexistence challenge to become a source of persistent operational drag - unreliable service delivery, high manual intervention costs, data quality problems that undermine decision-making - will find themselves at a structural disadvantage as competitors modernise and the regulatory environment tightens.

The cost pressure is already acute. BT and Openreach have both announced significant headcount reductions as the fibre rollout phase concludes, and the business moves into a period of operational consolidation. In this environment, delivering more with less is not aspirational - it is a commercial necessity. Intelligent management of the coexistence transition is one of the highest-leverage levers available.

The operators that will succeed are not those waiting to modernise before applying AI. They are those using AI to accelerate and guide the modernisation itself - treating legacy complexity as the first problem to solve, not the last.

Conclusion: Coexistence as Competitive Advantage

The coexistence problem will not be solved quickly. For most large telecoms operators, running legacy and modern infrastructure in parallel is a reality that will persist for years - not because of a lack of ambition, but because of the genuine complexity involved in serving millions of customers across systems that were never designed to work together.

The operators that manage this best will not do so by ignoring legacy complexity or by treating it as an obstacle to be overcome before real transformation can begin. They will treat coexistence as a discipline in its own right - one that requires intelligent tooling, strong data foundations, specialist delivery capability, and the ability to learn from what AI reveals about the real state of their operations.

The infrastructure that has served the UK for a century is changing. The challenge is not just to build what comes next - it is to keep what exists running reliably while you do.

About VE3 Global

VE3 is a dynamic technology consultancy dedicated to helping enterprises unlock the full potential of Cloud, Data, AI, and Digital transformation. We work with infrastructure operators, utilities, and large enterprise organisations on legacy modernisation, AI integration, data strategy, and programme delivery. Our work in the telecoms and utilities sectors spans OSS/BSS transformation, data platform builds, and operational AI deployment.

To discuss how VE3 can support your modernisation programme, contact us at ve3.global