Gandstream Handy Tone 386 - call transfer

Hi Guys,
here’s a curly one:
I have a Grandstream HT386 in use to interface to an analog phone (Uniden, wireless), all is well and working OK until I do a call transfer (using the flash button). the call transferrs OK, but after I hang up the analog phone (on the HT386) the HT386 doesn’t recover. i.e. no dialtone, no web access. It looks like the HT386 thinks it’s still busy on a call.

The only recovery from this is to remove power.
Asterisk seems OK, it recognizes the hangup (“extension exited non-zero”).

Any suggestions or ideas would be most apreciated.