Messaging can be confusing or downright aggravating these days. Google can't seem to figure out what it wants to do, and there are tons of alternatives all with their own insular user bases. You might just end up sending a text message, but thankfully text messages will get better thanks to RCS. T-Mobile was quick to deploy RCS messaging on its network back in 2015, and many Tmo subscribers are already using it without realizing. However, it hasn't committed to using the Universal Profile until now. Tmo now says it will start using the standard in the coming months.
All T-Mobile phones currently ship with RCS enabled via the carrier's internal version of the service. It works between Tmo customers, but other carriers use their own systems currently. According to Tmo's latest blog post, it will begin updating phones with the Universal Profile for RCS in Q2 of this year. It joins Sprint in supporting the Universal Profile, which was launched by the GSMA with Google's support in 2016.
RCS allows for advanced features that are not possible with SMS. For example, you can send longer messages, get read receipts, and send large files up to 100MB in size. Supporting the Universal Profile will eventually make RCS work seamlessly between carriers. All the other carriers need to add support for it, though. AT&T and Verizon have yet to confirm their plans for the Universal Profile.