[Serusers] prohibit gateway access - question

[Klaus Darilion]

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 at 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