ISDN Shutdown & Switching to VoIP/SIP Trunk: Process and Costs

Yes. Deutsche Telekom has migrated its network from ISDN to All-IP step by step since 2018; the nationwide migration of standard lines has been considered complete since the early 2020s. Pure ISDN lines are no longer provisioned in the public network. Telephony today technically runs as Voice over IP (VoIP) over the same broadband connection as your data traffic.

In practice, many mid-sized companies are still affected, because ISDN technology continues to operate on their premises: an older PBX with an S0 or S2M interface, analogue extensions, fax, door intercoms or special services. These devices do not speak SIP. They only keep working via a transition technology or have to be replaced. The actual task is therefore not the network shutdown, but the clean connection of your existing system to the All-IP network.

If you still use an old system connection, ask your current provider for the specific termination or migration date. Providers sometimes migrate legacy contracts forcibly with advance notice. An early look at the contract prevents telephony from being switched under time pressure and without testing.

There are two basic models for the VoIP switch. A SIP trunk replaces the former ISDN system connection. It delivers one or more number blocks and a defined number of simultaneous voice channels to your existing PBX. The system stays on site, and day-to-day operation barely changes for staff. This is the direct successor to the ISDN system connection and usually the obvious choice when you have a working, SIP-capable PBX or one that can be connected via a gateway.

A cloud PBX moves the phone system itself into a data center. On site you only keep IP phones or softphones; configuration, maintenance and updates are handled by the provider. This lowers the upfront investment, because no system hardware is purchased, and simplifies adding or removing users. In return, costs are usually per user and provider lock-in is tighter.

The decision depends on your existing setup. If your current system is modern, fully depreciated and fully used, much speaks for a SIP trunk. If a system replacement is due anyway, if teams work distributed or from home and features should grow flexibly, the cloud PBX is often the more future-proof choice. A neutral provider should calculate both paths rather than recommend only its own product.

Step one is taking stock. Record the system and its interfaces, the number of voice channels needed, all phone numbers and number blocks, and every analogue special case such as fax, franking machine, card terminal, alarm system, fire alarm or elevator emergency call. This list determines effort and cost and prevents nasty surprises on switch-over day.

Step two is sizing. The number of voice channels determines how many calls are possible at the same time. A rough rule of thumb is one channel per eight to ten active users; what really counts, however, is the actual concurrency behaviour, for example in service or sales teams. In parallel, you check whether the internet connection delivers enough stable bandwidth with low latency and whether voice has to be prioritised in the network via QoS.

Step three is number porting and scheduling. Existing numbers can almost always be carried over, so nothing changes externally. Porting is set for a fixed date, and older systems are connected via a SIP-capable router or media gateway. Step four is testing before live operation: inbound and outbound calls, direct dials, faxes, special services and behaviour during a power outage. Only after a successful test is the final switch made, ideally with a fallback level.

Most pitfalls are not the normal phone, but analogue special cases. Fax machines are the classic: VoIP transmits faxes reliably only if the entire path supports the T.38 protocol; otherwise transmissions break off. Where possible, switching to fax-to-mail or a documented fax procedure is the more robust solution. Modems, franking machines and some card terminals are also sensitive to VoIP and should be tested individually.

Safety-critical services deserve special attention. Elevator emergency calls, fire alarm and intrusion detection systems often communicated directly over the analogue line. For All-IP operation they must be checked, reconfigured or replaced with IP-capable communication modules. Here the specialist firm for the elevator or alarm technology and the telephony provider should be involved together early, because a non-working emergency call is a liability risk.

Another point is resilience. Pure ISDN often still provided emergency power feeding during an outage; VoIP does not. If the power fails, router, system and phones go silent without precautions. The remedy is an uninterruptible power supply (UPS) for the active equipment plus a contractually defined emergency forwarding that automatically routes inbound calls to a mobile phone or another site when the connection is offline.

Recurring costs are composed differently than with ISDN. You usually pay a base fee for the SIP trunk or cloud PBX, the number of voice channels and the call volume, often as a flat rate. Typical ballpark figures per voice channel range from low single-digit to mid double-digit euros per month, depending on the flat-rate scope [to be confirmed: actual tariffs depend on provider and volume]. Cloud systems mostly bill per user. The figure only becomes reliable with the stocktake from step one.

On top of that come one-off costs. These include setup and porting, possibly a media gateway to connect the legacy system, new IP phones, a system replacement if needed, and adjustments to the internet connection, network and UPS. These items are project-dependent and should be listed transparently up front rather than surprising you at the end. A serious quote clearly separates one-off from monthly costs.

Savings are therefore set against transition investments. Realistically, the switch pays off over cheaper call and line costs and the elimination of old maintenance contracts, not overnight. More important than the last euro is usually operational reliability: that telephony and special services work on the cut-over date and that a dedicated contact is reachable when something goes wrong. ITS AG handles such migrations for mid-sized business customers with dedicated contacts instead of an anonymous hotline, and with telephony over its own data centers in Germany.

For management, predictability matters most. The migration should happen with a fixed date, a clear cost breakdown and a defined fallback level, not as an emergency measure shortly before a forced switch. It is important that numbers are retained, critical special services such as the elevator emergency call are secured, and a binding contact for incidents is named. Telephony is business operations; a silent line immediately costs revenue and trust.

For IT leadership, technical viability is the priority. The internet connection and network must transport voice reliably with low latency and prioritisation, ideally with a second line or mobile backup. Security is part of it: SIP credentials, encrypted signalling and protection against toll fraud. Those who plan, document and test cleanly here turn the forced shutdown into a clean modernisation rather than a permanent construction site.