AT&Ts gateway worked for me around 2019. I was sending it from a domain that was set up with all the anti-spam things you have to set up to be a good sender (whatever rules so GMail etc don't dump all your messages right to spam, they are recorded but I don't recall them). It's possible that GMail was blacklisted because a lot of spammers had a ton of Gmail accounts.
Edit: Super important. You can turn that feature off per phone number. You (or your recipients) may have told att not to send them
Edit: Super important. You can turn that feature off per phone number. You (or your recipients) may have told att not to send them