Fiber is just a media and not a protocol or trunk type. I am assuming you are getting a SIP trunk from AT&T over the new fiber handoff.
When you say you tested outbound, how did you confirm it went over the SIP trunk rather your existing PRI as SIP trunk needs to be configured for it to be useful, plus dial plan changes are needed to route specific numbers via it.
As to inbound it all depends on your dial plan, do you require translations from dialed number to internal numbers, etc.
Since UC500 is based on Cisco CME can you first provide "show run" from the router?