We have tried several VoIP ATA's behind a Firebox II with a whole range of rules in the firebox but cannot get past the 5min32sec NAT dropout issue.
That is, the Firebox terminates the NAT session after 5 minutes 32 seconds and the call (UDP - RTP) drops out.
Would love to hear your feedback. (And yes, we know we should just replace the retired firebox - but can't in this case.)