...seems like I now need to register the number with T-Online.
Here I have my doubts. Me too was looking through the manual and I missed the page where the SIP settings would be explained. T-Online is a tough competitor, and I don't think they facilitate access to other companies which basically take their business away. As you have a sipgate username and password, you can log in on their webpage and look at how two devices need to be configured to have your VoIP working:
X-Lite 4
Nokia VoIP client
Unfortunately the linked content cannot be viewed without being logged in, but it is a bit more than the username and password you'll need to enter to get a working VoIP system using plain vanilla SIP (which is what you want to do). After all, by having a (normal) SIP-VoIP number registered, you can be reached on any host in the internet, wherever you want to be reached. Good stuff, just takes some efforts to set it up initially.
I need to login to "DSL Telephony". Anyone any idea how I do that?
This I'd assume will 'only' take you to the Telekom version of VoIP... sorry. By trying to make life easier for their own T-customers, they make it more difficult for anybody else.