nat=yes
Calls from me to the outside world work fine, audio goes both ways, everyone jumps for joy
Call to me from the outside world don't work, no audio either way, (ominously I don't see any data coming from the SIP provider either) but my RTP traffic is sent to the wrong host.
nat=no
Calls from me to the outside only have one way audio. The outside can't hear me, but I can hear them
Calls to me from the outside world don't work, exhibits the same behavior as above.
So in summary, if nat=yes I can at least make outgoing calls without any problems, I just can't take calls, but if nat=no then we're foobar'd all over :)