[Serusers] About the Inter SIP Proxy Problem

Dear ALL I have two SIP Proxy servers , One is SER, another one is CSPS (cisco) When I call a phone number that is belong to the CSPS routing, using the SER subscriber, the packet form SER to CSPS will get a problem with "400 Bad request" . (It's ok, If the UA in CSPS call to the UA in SER) My architecture is : UA1----SER----B2BUA---RADIUS | | Internet | | CSPS--UA2 Logs of B2BUA : 20060412-144443.276 [57396144] WARN bcWorker: uaState/UaStateRinging.cxx:133 UaStateRinging::Received 400 20060412-144443.276 [57396144] INFO bcWorker: controlState/StateCallReady.hxx:98 StateCallReady::bye was called, incomplete transaction Does anyone know how to solve it? thanks AR.