There's technical violations of confidentiality and then there are ones where your expectations of confidentiality are violated.

I would imagine it highly likely that you signed a release of information from your T to your group facilitator and vice versa (I did when I was in group therapy and had a different individual T). Did you exclude any content, such as "if I leave a voice mail message cussing out the group t . . . ?"

If you did sign a release of information, then there can't be any violation of confidentiality because of course you agreed they could share information without any restrictions.

I could understand why it felt like a violation, because you didn't think that T would have her listen to the message. I think my first instinct would be to feel betrayed-- and that's a powerful feeling for me-- I would have at least wanted to know that or been asked ahead of time, even with a signed release. If I step back from it (easy for me to do, because I'm not you), I could see why your T wanted to let you group T know how you felt, and she could talk talk talk to her about it or -- to be more accurate and less filtered, have her listen to the message. I think your T was trying to do the best she could to communicate with your group T, which after all was why they collaborate together. It wasn't for gossip or prurient matters, but to try to resolve this as soon as possible, so what happened wouldn't interfere with your therapy.