This has been a pain in my rear as well. Started in February for me. I've experienced the same thing you have, exactly. I tried everything...I changed my notifications back to email and set up a filter to forward the message as a text. at THAT point I discovered that the email address noreply@geocaching.com was being rejected by the tomail.net server. I ran other tests to be sure.
Along the way I sent several (3) tickets to our geocaching overlords. Each time I got the autoreply that they received my help request, and all three times that help request disappeared into the depths of the deathstar...uh...I mean HQ with zero response. Why is this?
So, today, being that I seem to have a bunch of time on my hands I reached out to T-Mobile directly. As fate would have it I connected with Colby, and after describing the problem it turns out that he is a lapsed geeocacher! Awesome, that makes this easier. After explaining in detail and some tests he tells me that geocaching.com has seemingly been added to a blacklist (see the attached screenshot of the texts).
I await his reply on Monday.
Eric1701