SIP client registration issue

I’d like to register SIP clients through SBC into IPPBX.

 

(access side)  SIP client - SBC – IPPBX (core side)

 

The first client registration works without any problem, it can start and receive calls and I can see it in the SBC registration cache.

When the second client try to register, the IPPBX  -which is a Cisco CM in this case-  sends back 404 Not Found message. Inside the captured packet the following message is:

 

Warning: 399 CUCM10PUB "Line not configured"

 

I think the IPPBX doesn’t accept the second client REGISTER because it arrives from the same source IP address and port as the first: it is the SBC sip-interface of course. So the second client REGISTER is handled by the IPPBX as if it belong to the first and registered client. But how the SBC should be configured in this case?