No Call diversion with SIP Phone (FBF 7170)

[nightlyflyer]

Hi,
I have a FBF7170 with ISDN line. I use both FON1,2,3 & a SIP VOIP-phone.
Making normal calls works well with FON and SIP phone.

Call diversion (Rufumleitungen) work well with FON1 connected analog phone:
*111#*#*21*nnnnnnnnnn# works (nnnnnnnnnn is a real phonnr)
*111#*21*nnnnnnnnnn# works,
*21*nnnnnnnnnn# works too.

But when I try to do the same using the SIP-Phone, calling *111#*#*21*nnnnnnnnnn# (as SIP call) just gets me a busy tone.

How can I enable Call diversion using a SIP phone??? What to dial?

Thanks, NF

(FRITZ!Box Fon WLAN 7170 Firmware version 29.04.82)