I don’t know anything about Ribbon SBCs, but there should be somewhere to create and configure a trunk that is most likely internet based, and you will need to whitelist the telco vendor ip’s etc. , then create the trunk which you will have options for digest authentication or even registration of an ip based trunk. Then you will create dial plan and use GHM (potentially) , to route your signaling/media.

Can you pull a pcap from the ubiquiti? Maybe run it through a Wireshark Voip filter? What does the call ladder look like? What do your traceroutes look like on a basic level, and where is the voip server in this scenario? Are you troubleshooting something in this scenario?