How to make a certain prefix be added to the original callerid? For example, the callers extension number is 555, the subscriber on the opposite side of trunk was defined as 77555 ??? 505 as 77505, etc.
inbound route has a spot to add a prefix if the call is coming through a trunk.
Take a look at the dial plan in the phone. You may also be able to create a new outbound rule that can prepend numbers, but in either case, there will need to be some concise rules to define the conditions of when to add so that not all calls have it added.