I spend the last many hours with support, my ISP, fiddeling with VPN etc trying to get my ip-telephony working – and guess what – we finally succeded – it looks like all the trouble was due to the use of the SIP protocol that seems to have some generic problems with firewalls.

There is another protocol for IP telephony, called IAX, this was what we tried in the end – I found a simple piece of software that used this protocol on Windows:
It can be downloaded from here.

This means that I can start to join teleconferences – yes!