From the old phone system to Microsoft Teams: process, duration and pitfalls

The old phone system still runs, but the maintenance contract keeps getting more expensive, ISDN was switched off long ago, and everyone working from home uses their mobile anyway. Sooner or later the question comes up whether Microsoft Teams can replace the system. It can — but getting there is a project, not a switch.

This article describes the process: what happens in what order, how long it takes and the three places where it gets stuck in practice. What Teams telephony is technically and what it costs is covered on our page about Teams telephony.

What the switch actually means

Call control moves from the server room into the Microsoft cloud, and Teams becomes the endpoint — on the computer, the mobile, the desk phone. Hardware may well stay on site: a session border controller or a gateway for special cases can remain in the building. Existing numbers can usually be taken over; whether and how is what a check before the project establishes.

Worth knowing: a Microsoft 365 licence alone does not include telephony. You also need a Teams Phone entitlement and a route to the public telephone network. There are three variants, differing in price and freedom.

• Calling Plans: Microsoft is the telephony provider itself — simplest, least flexible
• Operator Connect: a provider from the Microsoft programme is connected without your own hardware
• Direct Routing: connection via a certified session border controller, giving largely free choice of provider — and the route that best accommodates special cases and existing contracts

Which variant is cheaper depends on calling behaviour: number of users, call volume, international share, existing contracts. That is a calculation, not a rule of thumb — and it belongs at the start.

How does a migration work?

In five steps, of which the first matters most. Skip it and you pay for it later in edge cases.

• Inventory: which numbers exist, and what special functions hang off them — hunt groups, queues, fax machines, door intercom, alarm systems?
• Design: who gets which number, how do deputy rules work, what happens outside business hours?
• Build in parallel: Teams is set up and tested while the old system keeps running
• Porting: the numbers move to the new connection on an agreed date
• Follow-up: the old system stays live until the switch is confirmed, then it is removed

Parallel operation is not a luxury, it is the fallback line — with one limit worth knowing: the switch is reversible only up to the porting date. Once the numbers sit on the new connection, going back means porting again, with its own lead times. That is why testing happens before that date, not after.

How long does it take?

The plannable part — inventory, design, setup, testing — depends on the size and the special cases of the organisation. The harder part to plan is number porting: it runs to an agreed date but requires complete and correct details — number holder, authorisation, number blocks, existing contracts. Most delays arise there, not from deadlines.

In practice: check the master data first, then apply for the date, and plan the rest around it. The porting itself is carried out by the receiving telephony provider, not by us; we coordinate the process.

The three places where it gets stuck

They rarely have anything to do with Teams and almost always with what has grown around the phone system.

• The analogue leftovers: fax, door intercom, alarm system. These often sit on the same line and each needs its own solution, found before the project starts.
• Safety-critical lines do NOT belong in the migration. Lift emergency calls and fire alarm systems are governed by their own rules and require a connection that holds during power and internet outages — usually via a separate mobile route. These are handled separately and signed off by the relevant specialist firm.
• The network: voice forgives no delay. Where Wi-Fi, internet connection or network configuration are weak, telephony is where it shows first — measurable as latency, jitter and packet loss. That check belongs at the beginning, including what happens during an internet or power outage and how an emergency call is placed with the correct location.
• Habit: someone who has used the same handset for twenty years does not need training on Teams, but on making calls with Teams. That is a two-hour difference — and the difference between acceptance and rejection.

What is better afterwards

The main gain is not the price but that telephony stops being a separate system. A call, a meeting and a chat run through the same application; presence follows the calendar, and call handling can be configured once to follow that status — instead of someone setting a diversion case by case.

That is something to build on. Caller identification from your CRM shows the customer name instead of the number, and an AI phone agent answers calls that would otherwise end up in a queue. Both are separate building blocks with their own requirements for licensing, interfaces and data protection — sensible as a second step, not as a by-product of the migration.

Is the switch worth it for us?

Three questions usually answer that: is the maintenance contract for the old system running out soon? Does a meaningful share of your staff work mobile or from home? And do you use Microsoft 365 anyway? Three times yes means: it is worth doing the maths — with licences, connection and special cases on one sheet, not with a rule of thumb.

We support such migrations from inventory to porting, from Singen and within roughly 100 kilometres. If you would like to know what this means for your system, talk to us — or simply call, which seems fitting for this topic.