Astnow 1.7 with fpbx 2.8 cant answer a ringgroup


i setup a trixbox and cause problems a *now 1.7.
There i created a ringgroup and added several phones to it.
when calling the ringgroup, it is not possible to answer the call.
the called phones stops ringing, the phone hookup does not hear anything.
the caller still hears the ringtone.

i tried already other phones (normaly snom 370, the other testphones was an aastra 6751 and a old grandstream bt100.

after hangup of all phones, there are still two channels in use.
This channels can not hang up from asterisk -r.

as it happens with two different asteriskversion and different endpoints
i assume this is a problem of asterisk or the network.
so i tried also connecting the phones to an other switch, so change.

if i call the extenstion directly it works well.

where can i search for a solution?

best regards