[SR-Users] Topoh hiding prefixes

Hi folks, I have a topology like this: UA -> kamailio -> PSTN GW. When I'm placing calls to a PSTN I have to append prefixes so the PSTN GW knows how to route the call. So the request-uri is <prefix><number>@<gateway_ip> e.g. 999123456(a)192.168.1.1. When the UA calls to the GW a "200 OK" message contains a contact header with 999123456(a)192.168.1.1. This is a problem because I don't want my customers to see gateway prefixes. The Topoh module only hides IP addresses or the domain part. Is there any way to remove prefixes. This document (http://www.kamailio.org/events/2011-Cluecon/DCM-kamailio-security.pdf) stated "encoding IP and prefixes can be set via parameters". Can I remove prefixes from the contact in any other way than some contact header substitutions? Thanks Efelin