Incoming DAHDI forwarded to outbound IAX - one-way audio

Hello all

1.4.29 with TDM410P (box A). Incoming DAHDI call is dialled straight back out over IAX to another Asterisk box, also 1.4.29 (box B). Result: caller can hear callee, but callee cannot hear caller. Box A is behind NAT (box B is not), but I’ve never seen one-way audio with IAX before.

Can anyone help?

Thanks

Tom

box A extensions.conf:

[zap1] exten => s,1,answer exten => s,n,Dial(IAX2/***:***@***.***.***.***/252) exten => s,n,Hangup()

box B iax.conf:

[***] type=friend username=*** secret=*** context=sip host=dynamic trunk=yes disallow=all allow=gsm

box B extensions.conf:

[sip] exten => 252,1,Dial(SIP/611530)

Box A console:

== Starting post polarity CID detection on channel 1 -- Starting simple switch on 'DAHDI/1-1' [Feb 17 10:35:02] NOTICE[2929]: chan_dahdi.c:6643 ss_thread: Got event 17 (Polarity Reversal)... -- Executing [s@zap1:1] Answer("DAHDI/1-1", "") in new stack -- Executing [s@zap1:2] Dial("DAHDI/1-1", "IAX2/***:***@***.***.***.***/252") in new stack -- Called ***:***@***.***.***.***/252 -- Call accepted by ***.***.***.*** (format gsm) -- Format for call is gsm -- IAX2/***.***.***.***:4569-193 is making progress passing it to DAHDI/1-1 -- IAX2/***.***.***.***:4569-193 is ringing -- IAX2/***.***.***.***:4569-193 is making progress passing it to DAHDI/1-1 -- IAX2/***.***.***.***:4569-193 stopped sounds -- IAX2/***.***.***.***:4569-193 answered DAHDI/1-1

Box B console:

-- Accepting AUTHENTICATED call from ***.***.***.***: > requested format = gsm, > requested prefs = (gsm), > actual format = gsm, > host prefs = (gsm), > priority = mine -- Executing [252@sip:1] Dial("IAX2/eastburnin-2165", "SIP/611530") in new stack -- Called 611530 -- SIP/611530-000004ce is ringing -- SIP/611530-000004ce is ringing -- SIP/611530-000004ce answered IAX2/eastburnin-2165