One thing you could do is utilize Call Admission Control for this, you would assign the SIP trunk to Device Pool with LocationA, then each of your remove location and your main location would be in their own CUCM LocationX,Y, etc with specific bandwidth to/from LocationA allowing X calls. This way if locationB has exhausted all of their allowed bandwidth to locationA the calls would be rejected. I personally don't recommend it as the benefit of centralized SIP trunking is aggregate trunking where you don't need as many channels, an example of a potential issue could be for example lets say you have sites in different time zones with busy hour at different times, limiting them to X calls would prevent calls even though the SIP trunk is not at full capacity. My recommendation would be to do trunking analysis to figure out optimal aggregate concurrent calls and size the SIP trunk accordingly.
