I've been with T-Mobile for over a decade because their customer service has been impeccable.

I've been with T-Mobile for over a decade because their customer service has been impeccable. This is after being an AT&T Wireless (Cingular) customer for nearly a decade and watching their customer service depreciate with each merger and acquisition. In fact, my last day with Cingular (AT&T Wireless) was due to my $50/month plan billing me over $300 a month for multiple cycles in a row which required me to call or walk into a store every single month. After four or five months of this I walked into a store (quite pissed) and demanded to know why my plan was so ridiculously expensive every month. The kid behind the counter just laughed at me and when I rose my voice he said he'd call the police. I walked out, walked across the strip mall to the TMo store and never looked back.

For the record, I also had Verizon when I worked with them (company phone) that had real spotty service - worse than my Cingular (AT&T Wireless) service. But, the company was a terrible place to work and so they will never received and ounce of my money.
https://www.cnet.com/news/i-went-to-buy-a-galaxy-s9-at-verizon-and-at-t-it-didnt-go-well/