[SR-Users] RTP streaming problem (No Audio)

Dear All, I am working on Kamailio (V 4.0), installed and configured on ubuntu (12.04) System and it runs on Private IP address. I have also configured Media proxy with the Server. I have two Kinds of SIP clients : One is SIP hard phones and Another is IMSDroid clients. But the problem is its strange behaviour, that i didnt get : --> When i call Between Two SIP Hardphones, calls are OK.But when i tried to call between two Soft clients (IMSDroid), the very first time is fine without any problems. After that first session ends, and again if i call between two soft Clients, there is NO AUDIO both the sides. --> Through SIP Hardphones Audio would OK, But once the first call between Soft-Clients(IMSDroid) ends , thereafter No calls will stream RTP packets through this SIP Hardphones also. But the SIP Session is successfully establishing between All the clients, but ends with No audio. I know that kamailio Server will not negotiate any RTP streams, its only a proxy server. But somewhere is going wrong, can anybody guess what may be the reason for No audio ? Any Help will greatly appreciate. Regards, Nandini