Win 2003 ROUTE Change

Hi All,
I have two IP in my main network interface on windows server 2003 which have SIP application.

1- 174.49.13.74
2- 174.49.13.75

I need to add the ROUTE when clients use 174.49.13.74 the server should communicate to the destinations via header of 174.49.13.74, and same for IP-2.

Diagram:
1- Client <---->174.49.13.74 <-----> AT&T
2- Client <---->174.49.13.75 <-----> AT&T

But the current issue is always my server communicating to the destination (AT&T) via 174.49.13.74 IP, even if client send via 174.49.13.75.

You valuable response will be highly appreciated.

BR,