T-Mobile reportedly has a 5G service ready to go in 30 different cities in the United States, but has yet to flip the switch because there are currently no 5G phones available for use with the network. The Un-Carrier didn't state all of the different markets the 5G equipment resides in, but as was the case when T-Mobile first announced its plans, those cities are understood to include New York, Los Angeles, Las Vegas, and Dallas.
T-Mobile CTO, Neville Ray stated back in 2018 that the company would have 5G running in at least 30 different US cities by the end of that year, even if commercial services weren't yet offered, and that seems to be the case. Without a way to verify the presence of equipment or a network, of course, one simply has to go off of the carrier's word on the matter.
The lack of information regarding what markets 5G equipment is currently deployed in puts a damper on hype, since people won't know whether to expect 5G in their area in the near future. Most carriers will likely be starting to offer commercial 5G service this year as 5G phones begin to hit the market, and they'll be finishing their initial deployments for coverage of most of their markets within the next year or so. Just how much 5G coverage is going to be available when T-Mobile flips the switch was not specified. Even if it were, that figure could easily change by the time it happens. In any case, by the end of 2020, it's pretty safe to assume that T-Mobile, and perhaps other carriers, will have 5G in most areas.
While there would normally be a caveat with that last paragraph, T-Mobile will be delivering 5G services over both higher-band, shorter-distance spectrum like other carriers, and over its own 600MHz waves. This recently-deployed spectrum boasts wide reach and good building and ground penetration, though its speeds can be far lower than with higher ranges. T-Mobile has circumvented this using network virtualization and other backhaul tricks, meaning that any customer covered by its 600MHz network could potentially see 5G go live in the near future.
All four major carriers in the United States have their own plans for how to roll out commercial 5G services, and some have already started to do so. Verizon, for example, plans to offer 5G in select markets on the Moto Z family of devices via a 5G-enabling Moto Mod, though it should be cautioned that the tech contained in that Moto Mod will likely not enable 5G on other carriers, even if you unlock a Verizon Moto Z and bring it to another carrier with that Moto Mod in place.
30 cities is a very good start for a 5G rollout, and one can only hope that T-Mobile will have these 30 metros well-covered when the switch is flipped. The first 5G phone on T-Mobile's network has yet to be announced, but it's quite likely to be a 5G-enabled Samsung Galaxy S10 variant, a device that's already made the rounds on the internet. Those who aren't exactly fond of Samsung likely won't have long to wait, since Qualcomm's flagship chipset for this year, the Snapdragon 855, can be purchased in a 5G-capable form.