Who owns the trunk
PSTN, SIP trunking, and bring-your-own carrier
"Bring your own carrier" means the PSTN contract stays with you and an SBC you operate sits between that carrier and the call control. It is not a single product.
| Call control | PSTN choices | Where you look when it fails |
|---|---|---|
| Unified CM | SIP trunk to CUBE or another SBC, MGCP or H.323 gateway, or a trunk to another cluster | CSS, route list, then both dial-peers |
| Webex Calling | Cisco Calling Plan, or a local gateway you manage for that location | The location's PSTN model first. Calling Plan has no local gateway. |
| Teams Phone | Microsoft Calling Plan, Operator Connect, or Direct Routing through a certified SBC | The user's connectivity type. OPTIONS only if an SBC is on the path. |
| Contact center | The same PSTN as the PBX, or the cloud's own numbers, or a certified SBC into Teams (the Connect model) | Entry point or dialed number, then the carrier leg |
A SIP trunk is still RFC 3261 plus the trunk's own profile: early or delayed offer, transport, and which numbers are accepted. Two carriers at the same preference will split calls. OPTIONS can busy one of them out.
The Logicalis marketplace listing describes Teams Calling with Webex Contact Center through Direct Routing and CUBE. That is a published partner pattern, not a measurement and not the only legal design. Microsoft's contact-center article is the model list: Connect, Extend, and Unify.