I have a suggestion:
With this already 22 channels, i suggest you employ RF Frequency Hopping, with the following split…
BCCH – 15 Channels
Guard Band – 1 Channel
TCH(MA List) – 6 Hopping Channels (1X1 hopping on the MA List)
The bottle neck is there is that as the network grows,you will have to trade off between voice quality and capacity.