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Jaimin18's avatar
Jaimin18
Copper Contributor
Oct 01, 2026

How to transfer an external SIP call directly to a Teams user's voicemail?

Hi,

I am working on an external SIP-based reception platform that connects to Microsoft Teams through Direct Routing.

My requirement is to transfer an external call directly to a Teams user's voicemail without ringing the user's Teams devices.

The current flow is:

External Call → Reception Platform → SIP/Direct Routing → Microsoft Teams

A normal SIP transfer to the Teams user's SIP address rings the user. I need the call to go directly to their Teams voicemail.

I have been investigating SIP REFER/INVITE and found references to using:

sip:email address removed for privacy reasons;opaque=app:voicemail

I also found information about a User-to-User SIP header, but I am not sure which approach is officially supported by Microsoft Teams Direct Routing.

I have tried the opaque=app:voicemail approach, but I have not been able to get the call to reliably go directly to voicemail.

Could someone please clarify:

  1. What is the officially supported way to transfer an external SIP call directly to a Teams user's voicemail?
  2. Is opaque=app:voicemail supported with Direct Routing/SBC?
  3. Is there a specific SIP header or URI parameter required?
  4. Should the SBC modify the REFER or the subsequent INVITE?

Any Microsoft documentation or example SIP messages would be very helpful.

Thanks!

microsoft teams
SIP Phones
Voicemail

2 Replies

  • Jaimin18's avatar
    Jaimin18
    Copper Contributor
    Oct 05, 2026

    Thanks for the information. The Call Queue approach is useful as a workaround, but in our case we need to dynamically transfer calls to different users' voicemail from our external SIP platform.

    Do you know if there is any Microsoft-supported SIP header or URI parameter for Direct Routing that can send a call directly to a specific Teams user's voicemail?

  • DennisMatl's avatar
    DennisMatl
    Brass Contributor
    Oct 05, 2026

    I have never tried it through the SBC. We have had issue with add ons to an INVITE getting stripped as it passes through the various systems.
    We just created a Call Queue in Teams admin and used that to route that number to a users voicemail.
    We only needed to do that for a few weeks and that was the easiest solution for our situtation.