PBX A - IAX Config
[dynamichost]
type=friend
host=dynamic
context=local   ; Default context to request for calls to peer
callerid="Some Host" <(256) 428-6011>
nat=yes
requirecalltoken=no

Extension
exten => _X.,1,Dial(IAX/dynamichost/${EXTEN})

PBX B -IAX Config
register => dynamichost@69.59.158.114

[dynamichost]
type=friend
host=dynamic
context=local   ; Default context to request for calls to peer
callerid="Some Host" <(256) 428-6011>
nat=yes
requirecalltoken=no

Extension
[local]
exten => _X.,1,dial(ZAP/g1/${EXTEN})