Hi NYC78,

fellow German here.
Assuming that you're living in Germany your psychotherapist must be a member of a professional body ('Psychotherapeutenkammer'), assuming that she is a "psychologische Psychotherapeutin". There are different organizations, depending on the region where you live.
If she is a (medical) doc offering psychotherapy, she should be a member of the regional "Ärztekammer".

All of them have a professional code of conduct which they are obliged to follow. I'm not allowed to post URLs (yet) so you'll have to google Psychotherapeutenkammer & Berufsordnung & [Your Bundesland]

They vary slightly but all of them have in common, that premises for practice must be separated from private areas.
I would think that this should include seperate phonelines for practice and for private use - in order to prevent exactly what has happened to you.

I can totally understand that you are upset and I too feel that this is a violation of your privacy, and this she should apologize for. I'm not sure however she needs to tell you who the person was who answered the phone and I would try to keep this separate from the fact that the organizational level of her practice is clearly not following professional standards.

She should definitely take the precautions necessary to prevent this from happening again, either to you or to some other client. And she should no longer fail to understand that she really crossed a line by violating your privacy. If she keeps insisting that there's nothing wrong in the way she organizes her practice/private life I would seriously consider looking for someone else. Since she shows such bad judgement with regard to privacy and even though there are clear rules by her professional body, I wouldn't be too confident that this poor judgement doesn't extend to other areas as well...

Hope this helps a bit.
Take care and all the best,
c_r