Getting regulatory approval for the spectrum is one of the hardest problems, the hardest non-technical one. T-Mobile agreeing to allocate some of their spectrum to SpaceX is something that will probably happen with other carriers later down the line. AT&T and Verizon in the US are probably going to explore similar moves if the T-Mobile plan works well (IIRC, Verizon has plans for something with Amazon Kuiper). Wouldn't be surprised to see AST SpaceMobile as a target of M&A if they have something that works.