Sorry I read your post too late.
So I could not give you the answer you have found yourself in the meanwhile.
If users use the webinterface of SipX for setting up call forwardings there
is no need of setting call forwardings on the phone.
Once there was a problem with SipX and snom phones that bro
Thanks Tony, that did not work but it set me on the track to finding the
solution. Which was using :
redirect_time$
I am surprised I am the only one in the world having this problem as it
basically disables the use of forwarding
on the Snom