asterisk users - Sep 2004 - Grandstream x Asterisk 1.0 RC1 x VOIPJet

Sirs/Ladies,
Not sure if anyone saw anything like that before...
I was playing with an Asterisk setup with a Grandstream BT101 (1.0.5.11) 
and www.voipjet.com (IAX2).
The other devices I have home (Sipura 3k and DTA-310) seem to work just 
fine, but the Grandstream seems to suffer from one-way voice (remote end 
can't hear me).
The only workaround I found so far (have not spoken with VOIPJet supoprt 
about this), was to configure the GS entry in sip.conf to disallow=all 
and allow=ilbc, while iax.conf has disallow=all, allow=ulaw mulaw and 
ilbc (seems the call is established in this case, with the GS leg in 
ILBC and the voipjet in ulaw).
Should I assume this would force the Asterisk to do the transcoding to 
any other end-point that doesn't handle/prefer ilbc ?
What about DTMF (I've changed to 'SIP INFO' in sip.conf and the 
grandstream web config) ?
[], <O-O>