How do I file a complaint?

I just called ATT business mobile services and they connected me to someone probably from another country - like the Filippines, it sounded like.  Strong accent and very hard to understand.  When I asked about a 3rd line, she tried to hard sell me to sign up right then and there.  Told her I would think about it.  Then asked to cancel my $8.99 phone insurance.  She then put me on hold forever until I got tired of waiting without any response and hung up.  I think it goes against their scoring if we downgrade service while on the phone with them on a call.  This was horrible service.  Was on the phone over 40 minutes; could not understand her well, and now I have to call again to get this resolved by a more honest employee.  I wish we would get automatic emailed or texted questionnaires about each call we submit to them.  Then these sort of things wouldn't happen as much.  It's very common.  I used to work at a call center and that's what people do.