[Serusers] prohibit gateway access - question

Hi! I've got a small problem and don't know how to solve it best, so I would appreciate your comments: I want to allow my ser users to call any IP destination for free and always, but I want to restrict access to the PSTN. Therefore, I authenticate, account and check if user is in PSTN group before forwarding to the gateway (local GW or the GW of an PSTN termination provider). But, for example if one of my users call 1234567(a)anydomain.com and this domain resolvs to the IP address of the gateway, the request would be forwarded to the gateway, and the GW would accept the call as it comes from a trusted SIP proxy. How can I prevent this? regards, Klaus