Forum Discussion
MS Teams Direct Routing - Internal call transfer failure
I can confirm this behavior, just that REFER triggered transfer is most common way for it and only fully standardized one. If your SBC is capable to terminate REFER note that MS still clearly indicates in Refer-To where to send INVITE back:
REFER-TO: <sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689><sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689>sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689</sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689></sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689><sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689>
</sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689>
and making sure you fill RURI with Refer-To and Contact uri-host correctly you will see MS does rest of the job nicely for REFER case too.
It shows where to send INVITE to, but it doesn't have a DID or username in SIP R-URI so SBC can't figure out where to dial to. SBC uses PSTN-type dialing, so it needs some LineURI or DID to send INVITE to, like +44XXXXXXXX@sip.pstnhub.microsoft.com. Otherwise it can't decide what to put in SIP INVITE method as 'To' field. I've tried to update SIP INVITE to whatever is in REFER-TO header and send it back to Microsoft, but I was getting a 400 Bad Request back.
REFER-TO: <sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689><sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689>sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689</sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689></sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689><sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689></sip:sip.pstnhub.microsoft.com:5061;transport=tls;x-m=8:orgid:fce2158f-0e99-4ff7-a0b3-5b6b87d16689>
There's no user part in this SIP RURI, only host. What should be put in To header? Contact header is obvious, it's our SBC Contact.