Initially SpaceX and T-Mobile will only be offering text and SMS service in and around the USA. Voice and more data intensive applications would be a future update, most likely when they have a large portion of their new satellites in orbit.
At present, SpaceX hasn't even shown a functional proof of concept and Musk even acknowledged that SpaceX currently doesn't have a launch vehicle capable of carrying the large V2 satellites that are intended for use for cellular. The cell service is not going to use the existing Starlink satellites. The Falcon9 rockets aren't big enough for the new V2 and SpaceX's Starship, which would be carrying them, doesn't even have clearance to begin operating by the US government agencies. The cellular capable satellites have a 25 square meter antenna array on them, are larger, and weigh considerably more than the current Starlink satellites. (https://spacenews.com/spacex-and-t-mobile-partner-for-direct-to-cellphone-satellite-service/)
SpaceX also hasn't even given any indication as to how many satellites they'd need to get into orbit before they could offer anything approaching reliable service, especially outside of the USA. To have partial and intermittent service, they'd still need to launch a considerable number of satellites and they're likely focusing on areas that T-Mobile serves first. That could easily take months if not much, much longer. SpaceX is planning to launch almost 30,000 new Starlink satellites. Building and launching 30,000 satellites will cost an immense amount of money. He'll be monitizing it one way or another.
I'm also extremely curious how it'll work with standard cell phones. I can only imagine the phones would have to use a lot more power to get the signal to low earth orbit. I highly doubt any of the cell phones in existence have any kind of safety standards for low earth orbit use.
People complain about signal issues with terrestrial towers. They'll likely complain a lot more about satellite service. The cellular carriers and device manufacturers wouldn't be able to troubleshoot or offer support for satellite services and unless Elon wants to start a telco, I can't see him opening up call centres to offer tech support for cellular users. He offers poorly rated support for Starlink users, but those customers pay a lot of money for their equipment and service. How much more would someone be willing to pay on their cell bill for spotty satellite access? Musk isn't going to be offering the service for free.
There are still so many unknowns. I'd rather wait and see what SpaceX and T-Mobile are able to achieve before jumping on the bandwagon.