This bug was closed on the suggestion that senders manipulate a message to force MMS delivery:
That won't work in my situation. The unthreaded SMS messages, in my case, have been received and grouped properly on at least fifteen other Android phones and are know to work properly for iPhones on ATT and Verizon which leads me to believe the problem is with TMo's gateway and that Android has some built in workaround.
results in a text that reads:
firstname.lastname@example.org (me) / important / boo
Because the 400-5 part is variable, my iPhone 11 pro will not thread. And yet, no other carrier is similarly affected.
Please provide a better fix than "force MMS"
Best answer by tmo_amanda
I'd recommend PMing