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.
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.