@IBN Have you tried the diagnostic in the Microsoft 365 Admin Center yet? Navigate to Support > New Service Request and type "Can't make calls" in the problem description box. Choose the appropriate options for your scenario and enter the UPN of the affected user when prompted.
There's lots to unpack here, and lots of information missing. I assume because you mentioned SBC you're using direct routing? When attempting to make a call, what's the behavior of the client? Does it try dialing? Do you get any errors? How have you configured your voice routing policies? What about dial plans? Have you assigned any policies to the user directly? Or just set globally? What configurations have been made?
