[SR-Users] Problems with domain on registrations.

I have the current setup: SIP client <-> SER Proxy <-> SIP Trunk. My reason for the SER Proxy is that the SIP trunk only allows connections from approved IP addresses (such as my server with SER Proxy). My user on the SIP Trunk has the following authentication username@domain. I am able to register to the SIP Trunk as I'm setting up my SIP klient with username to username, domain to domain and then add my SER Proxy IP address as proxy. This works and means that I'm able to place calls to the SIP Trunk. BUT people cannot place calls from the SIP Trunk to my SIP client, due to the fact that on the SIP Trunk my locations is registered as username@SER-Procy-IP. So when people are calling me, the SER Proxy is looking for username@SER-Procy-IP and NOT username@domain, as I'm registered with. And then SER Proxy reports: 404 Not Found. Can anyone help me how to set this up correctly? I do not have access to the SIP Trunk, but I have all access to the SIP client and SIP Proxy.