The management of the TCH vs. PDCH is more complex : it is managed by MAX PDCH, MAX PDCH HIGH LOAD and MIN PDCH.
In your example, you are in high load, and i guess:
MAX PDCH HIGH LOAD is set to 5
like this, the 5 PDCH cannot be preempted for CS, either incoming HO or normal assignment.
if MAX PDCH HIGH LOAD is set to 3, your situation is not possible : with 15 TCH, you’re already in high load. Therefore only 3 PDCH can be allocated by the MFS. So PDCH 4 and PDCH 5 are going to be deallocated (even though there is no CS that is going to be handled on those TS right now)
You have a problem, but it should not be due to GPRS (what’s your value of max pdch high load ?)
I didn’t see any fault report about this. Can you raise this issue to the TAC ? I can’t say if this is due to AMR HR… maybe you can try to disable AMR for few hours, and see if it changes something ?
**For further study**
do you see the problem on all cells ?
what type of cells especially ? (umbrella ? concentric ? micro ?)
what type of handover is congested : internal, external, or intracell ?
when did the problem starts ? Since B9 migration ? Since activation of a feature ?
do you see a hourly correlation during the day between TCH HO Congestion and TCH NA Congestion ? What are those daily values ?
what is the value of the timer T7?
what is the value of T11, BTS_Q_LENGTH ?