[Users] Strange Problem involving PSTN call in

Hey Guys, I have had this very strange problem .. I have 2 PSTN gateways to allow for inbound and outbound calls. INBOUND calls from PSTN will reach one of these gateways (depending on the provider) and then directly routed to OPENSER. Gateways are cisco AS5350. The very strange thing is , on certain SIP-UAs that are being used outside (most of them ADSL users) , calls that come inbound via Gateway A can always ring the SIP-UA , calls coming inbound via Gateway B will not get a response. AND when i did a packet capture on the openser server , both gateways forwarded SIP requests inwards to the server and both are being processed . User Location is found and being routed. Both calls are being routed to the same UA , same port . So much for strange , anyone has any idea ? Regards, Sam