One Way Audio when Cancelling Attendent Transfer

WebRTC scenario.
call made

payload both directions OK.
call put to transfer - enter extension - attendent
external listening to MOH
webrtc user listening to ringback
cancel / cancel as remote party did not answer
one way audio - PBX to WebRTC is OK, no payload sent from web browser to PBX.

WSS in use - default flags in profile. RTP ports dont change according to PCAP.

any pointers would be great