Teams Voice

Yes, it can integrate existing PSTN number and third-party voice devices:

1. Direct Routing 
•    Allows your client to connect their own telephony provider or SIP trunk to Teams.
•    They can retain their existing PSTN number and use certified Session Border Controllers (SBCs) to route calls between Teams and their legacy system.
•    Ideal if they already have PBX systems, SIP trunks, or voice gateways from other vendors.
2. Operator Connect
•    To work with a certified telecom provider who handles the PSTN integration.
•    They can port their existing number to the provider and assign it to Teams users.
•    Easier to manage than Direct Routing, but less customizable.
3. Microsoft Calling Plans
•    Microsoft becomes the PSTN provider.
•    Your client would need to port their old number to Microsoft, which can take time and may involve relinquishing control of the number.
•    Best for small businesses without complex voice infrastructure.