you can create firewall rules (given that your firewall is in drop mode) who accept everything which does match the protocol you use (not sure it’s possible in your case though) or rules who accept packets based on “Layer 7 filters”.
I guess you want only VoIP to work? In this case, a few rules with layer 7 inspection are needed. I’m using it for QOS, but not sure it will fit all your needs : add accept rules for SIP, H323, Ventrilo, Teamspeak, Skypeout, skypetoskype.