One way it could happen is if you have different tipes of CU or whatever it is called by the vendor. For example on BCCH one could have EDGE CU which is with higher power for GSM but the rest TRX with normal amplifiers. If by any reason you have difference between TRX with the SDCCH and the one where the TCH were allocated there you have a problem. Also for common BCCH.
You should use an analyzer to see where the TCH for the call was allocated