What happens to your existing phone numbers when you move to Microsoft Teams Calling?

It's almost always the first question we hear when a business starts looking seriously at Microsoft Teams Calling: "What happens to our phone numbers?" The good news is clear and simple — your numbers come with you. Number porting is a well-established process in Australia, and with the right carrier managing the transition, it happens without your customers ever knowing anything changed.

Here's exactly how it works, what to expect at each stage, and the traps to avoid.

What is number porting?

Number porting is the process of transferring ownership of a telephone number from one carrier to another while keeping the number itself identical. Your customers keep dialling the same number. Your business cards and website don't need updating. The only thing that changes is which network carries the calls behind the scenes.

In Australia, porting is governed by the ACMA's number portability framework, which means all carriers — including your current one — are legally required to cooperate with a legitimate porting request. They cannot refuse to release your numbers.

Which types of numbers can be ported?

Standard geographic numbers (02, 03, 07, 08)

These are your everyday DDI (Direct Dial In) numbers — the kind that begin with an area code. Adelaide numbers starting with 08 fall into this category. Geographic numbers are straightforward to port. The process typically takes 2–4 weeks from submission to completion, and the vast majority go through without issue provided the account details match what the losing carrier has on file.

ISDN numbers (CAT-C)

ISDN lines — including ISDN2 (BRI) and ISDN30 (PRI) — use what's known as a CAT-C port, and this is where timing becomes critically important. CAT-C ports take 8–12 weeks from submission to completion. This isn't a carrier being difficult; it reflects the additional coordination required between networks to migrate numbers off legacy infrastructure.

The trap many businesses fall into is waiting too long to start the porting process. If you're planning to retire your ISDN service — which Telstra has been progressively decommissioning — you must initiate the port well before your ISDN disconnection date. If the line is disconnected before the port completes, recovering the number becomes significantly more difficult.

How does the porting process actually work?

When Caznet manages a number port for a Microsoft Teams Calling migration, the process follows these stages:

1. Letter of Authority (LOA): You sign a document authorising Caznet to port your numbers on your behalf. This also confirms your account details with the current carrier — name, address, and account number must match exactly or the port will be rejected.
2. Port submission: Caznet submits the porting request to the losing carrier through the industry porting system. The losing carrier has a set timeframe to respond and cannot unreasonably reject the request.
3. Port date confirmed: Once approved, a firm porting date and time is agreed. This is typically scheduled during business hours to allow for monitoring.
4. Redirect to Teams Direct Routing: On the port date, your numbers are redirected to Caznet's Direct Routing infrastructure, which connects your numbers into your Microsoft Teams environment. Calls that previously reached your ISDN or SIP service now route through Teams.
5. Verification: Each number is tested immediately after the port to confirm inbound and outbound calling is working correctly.

Parallel running — testing before you cut over

For larger migrations, it's often possible to run both the old and new phone systems simultaneously for a period before the final cutover. During this parallel running phase, your new Teams Calling setup is fully operational and tested, while the existing system remains live. This gives your team time to get comfortable with Teams before the old system is switched off.

The approach depends on how your current telephony is configured. In many cases, Caznet can provision new SIP trunks into your existing setup alongside the Teams infrastructure, allowing side-by-side testing without any disruption to live calls.

What can go wrong — and how to avoid it

Number porting is reliable when it's managed carefully. The most common causes of delays or rejections are:

• Mismatched account details. The name and address on your porting request must match what the losing carrier has on file exactly. A slightly different business name or a changed address that hasn't been updated with your current carrier will cause a rejection and reset the timeline.
• Starting too late. Especially for ISDN ports. If there's any chance your ISDN service could be disconnected before the port completes, you're taking an unnecessary risk with your numbers.
• Not testing before cutover. Going live without testing each number in the new environment is a false economy. A few minutes of testing per number before the old system is retired saves significant pain afterwards.
• Forgetting about fax numbers. Businesses that still use fax over ISDN sometimes forget these are numbers that also need to be ported or redirected. If you have fax lines, flag them at the start of the project.

What Caznet manages on your behalf

When you migrate to Microsoft Teams Calling with Caznet, we handle the entire porting process end-to-end. That includes preparing the documentation, coordinating with your current carrier, scheduling the port date, configuring Direct Routing to receive your numbers in Teams, and verifying each number after the cutover. You don't need to negotiate with your outgoing carrier or navigate the industry porting system — that's our job.

The result for your business: the same phone numbers, a modern calling experience inside Microsoft Teams, and a migration that your customers and staff won't even notice.