My voice setup is as follows:
CCM5.1---(SIP)---IPIPGW----(SIP)----SP's softswitch. In this case,i send all calls to this service provider and the SP routes to other networks. Calls to other SPs works ok, however calls to the terminating service provider don't return the ring out tones. A user dials, some silence then you hear a hello from the person you were calling. The SP has both GSM and landline. All GSM calls behaves this way but calls to some selected landlines are ok. Kindly point me to the right direction in solving this and the probable causes.