I am writing this post to document an issue and a solution that may be of use to others who come across it since I was not able to find any useful posts elsewhere that relate. I have two new factory-unlocked Moto G4 Plus phones that I purchased from a 3rd party and brought to T-Mobile to replace aging Galaxy S4 devices. A major reason for upgrading was gaining access to Band 12 and voice over LTE (VOLTE) coverage, which T-Mobile has been expanding.
The default setting under Settings:more:cellular networks:Enhanced 4G LTE Mode is "enabled," which allows the phone to use VOLTE. Coverage with this setting enabled was exceptional, with no dropped calls vs. my S4, which repeatedly dropped calls in certain areas. However, with the Enhanced setting enabled, SMS was mostly non-functional. Even though messages would say that they had sent, they would not be delivered to the recipient, and incoming messages wouldn't show up on the phone (even though they would arrive in my account, as verified by a DIGITS duplicated device with the same phone number). The SMS issue could be isolated to the Enhanced switch...enabled-SMS woudn't work, disabled-SMS worked fine.
The problem with disabling "enhanced 4G LTE mode" to make texting work properly was that it put me back to living with dropped calls.
Moto support was unable to identify the issue and wanted to factory reset the phone. However, before doing that, I contacted T-Mobile support a second time and worked with a technician who was eventually able to identify the cause, which was easily resolved. The cause turned out to be re-use of a dated sim card in the new phone. Although it generally worked, the sim card apparently did not fully support current Band 12/VOLTE features. By installing a new sim card, the issue was resolved, and texting now works using Enhanced 4G LTE mode.
Hope this information helps someone struggling with the same problem.
*Discussion moved from Other Android Discussions to Not Sold by T-Mobile Discussions - Marissa