What we know:
 <ata> -> NAT -> * works fine
 <ata> -> NAT -> * -> phone works fine
 <ata> -> NAT -> * -> 7200 pukes

brian added:
 <ata> -> NAT -> * -> Cisco 7200 (SIP) -> ISDN
 <ata> ---------------------------------> ISDN (audio ok)
 <ata> <--------------------------------< ISDN (no audio)