[OpenSER-Users] check_from() and calls from a PBX

Hi, I've a PBX (Asterisk) registered in OpenSer as user "pbx@domain". When Asterisk receives a call from PSTN it calls to OpenSer users with From: From: <sip:PSTN_number@domain> But of course, this can't validate "check_from()" since From username (PSTN_number) and auth username (pbx) are differents. As I solution I don't do "check_from()" if From username is "pbx" but of course, it can create security problems. Other solution is to force "From" username in PBX outgoing calls, but with this the callerid number will be "PBX" (that is not very cool for end users xD). Another solution is using "trusted" ips, but this is not valid for me since Asterisk is behind a NAT and dynamic IP, and because there can be multiples PBX for each domain of OpenSer and I don't want they to have privileges. Is there any other approach for this? Thanks for any suggestion. -- Iñaki Baz Castillo ibc(a)in.ilimit.es