One-way audio after assisted transfer with Nokia E63

Hi, my environment:

Asterisk PBX with freepbx
Around 60 snom phones (300,320,360,870) with latest 8.4.18 firmware
Around 50 Nokia E63 configured as voip extensions on wireless lan with latest firmware and sip stack


A=caller (internal or external)
C=Nokia phone

A calls B
B puts A on hold
B calls C
B transfers A to C
B hangs up

Now A can hear C BUT C can’t hear A
If C presses any key on the keypad, the call starts to work 2-way

This DOES NOT happen:
if B is not a snom phone
if C is not a nokia phone
if B uses blind transfer instead of assisted transfer
if B uses asterisk to do the assisted transfer (#)

All the firmware version (nokia and snom) are up to date
I tried to:
change all the possible options on the mobile phone (including codecs and dtmf) without luck
disabling NAT options on all the devices, even if NAT is not involved at all in my environment (all phones and PBX on the same subnet)

Thank you