How can I replace my legacy PSTN infrastructure with cloud-based SIP Trunking?
Replacing legacy PSTN with cloud SIP trunking is a five-step process. You audit the current setup, plan number porting, size your trunk capacity, run a parallel cutover, then decommission the old lines. For an enterprise in multiple countries, most of the effort lands in porting and emergency calling, which differ market by market.
The steps below match what most guides list. What changes at enterprise scale is where the time goes. Two steps, porting and emergency calling, are simple in one country and hard across several. Legacy networks are closing on fixed dates, the UK in January 2027. The task now is migrating cleanly across the sites you run.
Step 1: Audit what you have
Start by counting your active lines and documenting the PBX they terminate on. Note its model and firmware, and whether it speaks SIP or needs a Session Border Controller (SBC) in front of it.
Then export your number inventory in E.164 format. That inventory covers your DIDs (direct inward dialing numbers) along with main and service numbers. This list drives the porting plan, so accuracy here saves weeks later.
For a multinational, record which country each number belongs to. Porting is governed locally, so the audit is really a per-country inventory, not one big list.
Step 2: Plan number porting
Porting keeps your existing numbers when you change providers. It is the step most likely to set your timeline, because you do not control it alone.
Rules and timelines differ by country. Some markets port in days; others take weeks because of local validation and paperwork. Plan each country separately and sequence them, rather than assuming one global porting date.
Ask your provider for per-country timelines in writing. A provider that holds the carrier license in a market controls the port directly. One that resells a local carrier there depends on a third party outside your contract.
Step 3: Size your trunk capacity
SIP trunk capacity is measured in concurrent calls, also called channels. You need enough channels to cover your peak simultaneous calls, not your total user count.
Size it from real data. Pull call records to find your busiest hour, then add headroom for growth and seasonal peaks.
Step 4: Run a parallel cutover
Keep the old PSTN live while you test SIP alongside it. Do not port a single number until the new path is proven.
Test the scenarios that matter: inbound and outbound calls, your call routing, failover for business continuity, and emergency calling.
Emergency calling is the scenario teams skip and regret. Each country mandates how emergency calls are routed and how caller location is presented. Configure and test it per market before you cut over, not after.
Once the scenarios pass in a market, take the porting date for that country and cut traffic over.
Step 5: Decommission the old lines
Once numbers have ported and calls run on SIP, retire the legacy circuits. Cancel the old PSTN and ISDN lines per market, and remove any hardware the new path no longer needs.
Hold the cancellation until the port is confirmed and billing has moved. Cancelling early can strand a number mid-port.
What makes the enterprise version harder
Strip out the per-country detail and the five steps are ordinary. Add it back and two steps dominate the project. Porting and emergency calling are where a multi-country cutover slows down, because each market has its own rules and its own clock.
The simplest way to keep that manageable is one provider across as many of your countries as possible, holding the carrier license in each. That puts porting and emergency calling under one accountable party instead of a different local carrier per market.
Where Pure IP fits
Pure IP delivers PSTN replacement over SIP as a licensed carrier, available in 50+ countries. It owns the voice network in the markets it covers, so porting and emergency calling sit with one point of contact and accountability. A multinational runs the migration through one provider and one contract, country by country, instead of coordinating a separate carrier in each market.
Common questions
How long does a PSTN to SIP migration take?
It depends on scope and on the slowest country to port. Single-country migrations can move quickly; multi-country projects run as long as their hardest market takes. Plan around per-country porting timelines rather than a single estimate.
What happens to my phone numbers when I switch?
You port them. Porting moves your existing numbers to the new provider and keeps them in service. Timelines vary by country, so list your numbers in E.164 format and plan porting market by market.
How do emergency calls work after moving to SIP?
Each country sets how emergency calls are routed and how caller location is provided. Your provider configures this per market, and it should be tested during the parallel run before any number ports.
Before you start
Begin with the audit: count your active lines and export your numbers in E.164 format, with the country marked against each one. Then ask each provider on your shortlist for per-country porting timelines, in writing.
Talk to the team that runs PSTN-to-SIP migrations as a licensed carrier in 50+ countries.
