I have Verizon as my main carrier and was in the beta. I'm using a Samsung Galaxy S22 Ultra for my phone. I've sent a few texts from the T-Mobile sim card in service and sent/received a few texts through the satellites when out of service. Satellite texting send and receive seemed to take about 10 seconds per message which is quite a bit faster than my InReach was. I was able to have a normal conversation while out of conventional cell service.
The functionality seems to work about the same as my InReach. A clear view of the sky is a necessity and you have to be completely out of service before it switches. It seems like if you're hovering between one bar or service and none, there's not enough time for it to lock on the satellites. It only seemed to lock on if I was out of service for more than 5 minutes at a time.