FRITZ!Box Fon ata 1020 - only one SIP call at a time

I have FRITZ!Box Fon ata 1020 with firmware version 11.04.01. I cannot
make simultaneous SIP calls on Fon 1 and Fon 2. It does not matter if
the second call is incoming or outgoing, or which provider it is (I'm
using SMSLISTO/BETAMAX and VOIP.CO.UK). The box rejects the additional
incoming calls and sends secondary outgoing calls over PSTN. I've not
experienced this limitation before with other FRITZ! devices. I wonder
if the first call is hogging the codecs somehow, but I have no idea
how check or to fix this. Anyone had this problem or has some insight?

 

Maybe they can help:
http://www.avm.de/en/service/support/FRITZBox.php


if not then relax here:

http://digedag.di.ohost.de/fun/vehicles/slides/poliphone.html