Incoming fax via CAPI to Hylafax crashing SIP channel

hello! there is something i cannot explain - maybe some of you can:

we are running asterisk and hylafax. both using the CAPI interface of an AVM B1 controller. works fine.

but there is someting really strange: when a fax arrives, hylafax takes the according channel and starts receiving. i don’t notice anyting on the asterisk side about that (so it should be…). but when doing an call via asterisk an the CAPI interface, after 10 to 15 seconds fax-receiving, the sip (or isdn) channel starts to loose quality… 10 seconds later, the call is still connected but there is only some undefineable kind of nois to hear on both sides. also the fax-side seems to be disturbed… some faxes do arrive in this situation, some are aborting.

any ideas? just for information: doing 2 calls via asterisk over this interface, everything works fine. only receiving a fax via hylafax pruduces this strange problem.