You have to make sure the Cisco end of the trunk is administered as Internal. Beyond that, I was a UCM user but not an admin so I can’t offer much help. If the UCM is sending the streering digit plus the extension to the FreePBX, you need to use the FreePBX inbound route to send to a custom destination to strip off the steering digit. Drop into the FreePBX console mode and see what the log says as the call comes in from the Cisco. You’ll be able to see the number it’s sending you.
I also think you have to put a route in the Cisco so if it sees something like 85901 that it knows to route it to the FreePBX trunk. Its possible the Cisco can strip out the leading 8, but you’ll have to consult someone who does CUCM programming.