Can't send SMS (but can receive)

I recently switched my line from AT&T Prepaid to T-Mobile prepaid. I discovered today that for at least the past two weeks my phone has not been sending text messages, although they appear to have been sent successfully. I've also haven't been receiving all my calls and voicemails left for me. Upon contacting T-Mobile support via online chat, I was met with a refusal of the first representative to help resolve the issue and automatically assumed the problem was my device. He kept insisting that I go to a t-mobile store even though I let the rep know I just tried and failed to send a text from a different device using my SIM and it also showed as successfully sent but was never received by the other party. I was going to switch back to my previous carrier today to avoid any more of these issues with t-mobile but they charged my card a day earlier than they were supposed to!