It sounds like the Call Admission Control is stoping you as per the bandwidth and codec used from Locations and Regions. If you are using G711, you could change it to G729 and it will give you more bandwidth to play with.In turn, will allow more calls to go thru. If you introduce IP RTP Header compression, Callmanager will not know about it, so you will have to manually do your math and allow X numbers of calls with the calculations.