General W2k problem ( with AVM-Fritz!Box TAPI, Funkwerk elmeg TAPI )

Hello,
on my W2k-pro and another W2k-server, I have a general problem,
in opposite to a Vista-Business computer in the same local network,
using
(1) AVM-Fritz!Box TAPI
(2) Funkwerk elmeg TAPI
for real hardware devices ( Fritz!Box, elmeg CS 410 ISDN telephone )
which are connected to my computer.

I am using readymade sample applications
a) Microsoft Outlook 2000
b) "Soft Phone" ( "Phone" ) of http://www.julmar.com/tapi/
as these supports the address selection of TAPI devices ( necessary to
select an MSN for ISDN telephones ).
The other Julmar application "Makedrop" does not support address
selection

On W2k, with "Soft Phone"
With AVM TAPI, I can to "internal calls" like **1.
With AVM TAPI, I fail to call external numbers ( 0511-12345678).
With Funkwerk TAPI, I fail to call external numbers ( 0511-12345678).

On another Vista computer in the same network
With AVM TAPI, I can call external numbers ( 0511-12345678).
With Funkwerk TAPI, I can call external numbers ( 0511-12345678).

In all cases, a local phone district ( 511 for 0511 =3D Hannover, in
Germany ) is entered.

As W2k is a "classical" OS with which there should be no problem with
oldfashioned technologies like TAPI, I am surprised. One of the W2k
systems was freshly installed lately.

***************
Is this effect KNOWN ?!
So is there something I can to, so make (both) TAPI phoning work on my
Wk systems ?
Any trick or patch, which I can`t guess ?
I can=B4t believe that W2k was totally rotten for TAPI applications !
Nobody told ever in the Internet.
***************

The  AVM TAPI driver is officially unsupported by AVM, and
and I can=B4t expect unpaid official support by Funkwerk for such
special problems, as for anybody here WinXP is standard ( and it works
with Vista, of course ).

I know that I ask a few years "too late" as Microsoft now discontinues
to support W2k.Don=B4t tell me about using the Vista computer or
upgrading from W2k to WinXP, thats not an answer for me.

And from what I learned, TAPI is not leading edge technology for
Microsoft, so it might be discontinued "after" Windows7 ...so I look
for a W2k solution.

I found 2 other discontinued VoiceOverIP / SIP softphones ( Nero
Sipps, Telekom Internet-Telefon ) but due to its state to belong to a
discontinued service, I did not test them so far, successfully
( without TAPI ).

So any help is appreciated !

Sincerely
Rolf