Use Webex Edge for Devices with Optimized Join Experience and the endpoint will perform a native join instead of a SIP call through CUCM. This also allows the endpoint to leverage Hybrid Calendar for One Button to Push (replacing TMS-XE, if you have that deployed.) Note that this requires a Cloud Device Registration license, although it is included in most calling enterprise agreements.
Even with Edge for Devices (aka cloud-aware) any SIP URI calls would still route through CUCM. Joining a Zoom Meeting, for example. You can avoid that by going full cloud-reg with Webex Calling instead of CUCM. If that’s not desirable you should deploy Expressway and configure business to business calling. Entitlement for Expressway is included in all current on-premises calling annuity subscription offers. Note that Webex.com is exempt from Expressway Rich Media Session licenses but any other SIP B2B call would require them. RMS licenses are not cheap so get pricing from your reseller before settling on a path forward here.
As for doing it with CUBE, check out Domain-Based Routing Support on the Cisco UBE . It should be possible to make this work if CUBE has a public IPv4 address on the internet-facing interface (ie it will not be NATed) and is in flow-through mode; however, this does not align with Cisco’s Preferred Architecture which specifies Expressway as the internet edge solution component. You’re going “off the reservation” by using CUBE instead - be sure you can defend why to TAC and any other Collab engineer.
