After more checking, all of the AT&T texts that were 770 characters and above succeeded. When I check the Telnyx logs it shows that they converted to MMS and arrived correctly. Texts 760 and below failed intermittently.

Texts from Twilio didn’t convert to MMS. I assume it is the app that chooses when to convert and I was using Twilio from a 3CX PBX. My tests from Twilio were successful for a 950 character text, and a 1000 character text, which were 7 parts each, but were 50/50 success/fail between 600 and 850 characters.

I don’t have personal access to phones from T-Mobile and Verizon so I can’t do the more extensive tests from those, just what my co-worker has tried so far and reports from our customers that various larger texts fail at random.

MMS is coming through fine from what we have seen. Various customers receive images to their Telnyx numbers and haven’t complained about that. Just larger SMS messages.

Also, when I look at the logs from my cell phone it shows the tests all being sent as Text Message for the usage type. I don’t know if AT&T converts really large texts to MMS to help with the transfer, but it doesn’t look like they do for 2-3 part messages.