It looks like T-Mobile looks for ".xyz" within the SMS and will silent drop the SMS (though it will claim it is delivered). ".xxyz" works, "..xyz" or ".xyzz" does not. "xyz" works, so does ".xy".
I thought SMS didn’t have delivery receipts?
MMS has delivery reports too (I implimented support for it myself for mainline Linux Phones). It even has read reports, but no carrier seems to honor using it (which is why I didn't bother to impliment it).
I'm not sure if Android/iOS gives the user an option for it (which may be the source of confusion).
I'm not sure if SMS supports read reciepts, but I didn't think so. The MMS standard allows for read receipts ("MAY" not "SHALL"), I was unable to get it working, and I suspect it's due to no carrier support.
I was unable to get read receipts working at all, and I suspect it's because the carrier doesn't impliment it.
Also, Telegram seems to be much better supported on the Pinephone as of now, so that is what I generally prefer.