An old phone system can appear cheap because the main hardware was paid for years ago. The real cost often sits elsewhere: line rental, maintenance, separate call charges, unused numbers, engineer visits, ageing handsets and the operational cost of working around limitations.
Start with a complete telecoms inventory
List every telephone number, analogue line, SIP service, broadband connection, PBX licence, handset, support contract and recurring call package. Include devices such as alarms, lift lines, payment terminals and fax equipment if they still depend on a telephone line.
Businesses often discover services that nobody can confidently explain but which have remained on the bill for years.
Look for services you are paying for but no longer use
Old DDI ranges, spare lines, redundant hunt groups and maintenance add-ons can survive several office moves or supplier changes. Removing them may produce savings without changing the customer experience at all.
Do not cancel blindly, however. Confirm where every number and line is used before making changes, particularly where alarms or other operational equipment may be connected.
Include maintenance and support effort in the calculation
A legacy PBX can become expensive when replacement parts are scarce or only a small number of engineers still support it. Even if failures are infrequent, recovery can become slower and more disruptive.
A modern platform should have a clear support model, documented configuration and readily replaceable endpoints.
The cost of downtime is more than the repair invoice
If customers cannot reach sales or support, the impact is operational. Staff may switch to personal mobiles, call records become fragmented and managers lose visibility of what was missed.
Resilience planning should therefore be part of the financial comparison, including broadband failover, alternative routing and how quickly numbers can be redirected during an incident.
PSTN retirement turns some legacy costs into migration costs
Openreach says traditional PSTN phone lines are due to move to digital by 31 January 2027. Businesses that still rely on legacy services need to identify what those lines support and migrate them appropriately.
Leaving the work late can compress testing and procurement into a shorter window, especially when analogue-connected devices need specialist alternatives.
Compare like with like before switching
Do not compare only the headline cost of a SIP trunk with the rental of an old line. Include licences, handsets, connectivity, support, call capacity, number rental, setup work and any resilience requirements.
The right objective is a service that is commercially understandable and operationally suitable, not simply the lowest possible monthly figure.
Review your numbers, SIP service and phone-system options
UKDDI can supply UK business numbers, SIP delivery and compatible VoIP hardware. Start with the numbers and call flows the business needs, then choose the platform and capacity around them.