As far as I remember you need to add Cisco CMR on top of your meeting center so that you can dial in to the room using SIP URI's
Check the latest guide here
https://help.webex.com/community/webex-admin/content?filterID=contentstatus%5Bpublished%5D~category%5Bcmr-cloud%5D
Or if you have the on prem telepresence infra (TMS, Conductor, TS/MCU etc) you can do the CMR Hybrid (using your existing webex meeting center)
Hope this helps