Thanks a lot for your reply.
Unfortunately I still can't get it to work. I keep getting »…bitte versuchen Sie es später noch einmal.«
It is all about the SIP trunk username and password, right?
Username: telephone number with local prefix
xxx is the one email address I use for logging in at telekom.de, isn't it? I don't have one that ends with t-online.de. I use one with @web.de. Can this be the problem?
I have tried with and without :…@web.de, with firstname.lastname@example.org, with email@example.com - nothing works.
On my Mac I use the app Telefon by 64 characters. There username is also telephone number with local prefix and password is just the Telekom Internet password without :email !? And there it works. Can anyone explain to me what the difference is? Which password syntax will be right?