Call setup time is difficult to improve. But normal call setup time mobile to mobile is about 7 – 8s in GSM.
You could breakdown the call setup from the L3 message, so you could rectify the problem, where is the long time to take? is it on the radio side or core interuption side.
The problem I found like, the authentication occure very often (it must be tuned),some feature in MSC is activated, like IMEI check which take about 0.5s( could be tuned), RACH collision on radio interfaces (could be tuned)
Some feature on MSC could be activated like off air call setup to improve call setup time.
Another way is, the MTC number must in the same location (the MTC number must be the same location with HLR). Hope will help.