I've been a VoiceStream, then T-Mobile customer for over 16 years. There's been ups and downs in the relationship over the years. The most notable "down" was the $800 international roaming charge they refused to remove from my bill a few years back. Even though I really wanted to leave them then, a thorough cost analysis of the competition showed they really were still cheaper...
More recently, the Simple Choice plan they introduced last year which includes "free" international data roaming has ensured I stick with them for even longer. I travel quite a bit, so that + the wifi calling which works pretty much anywhere in the world has been a great thing.
I dropped them many years ago over a similar issue, except in my case I was almost 40 miles from the Canadian borders and picked up a Canaidam tower... They slapped me with a huge fee for international roaming.
This was late 90s, so I think a 10 minute call cost like $150!
A lot of the time, such charges come up because the customer was unaware, not because they used it. I once made a call from whatsapp not realizing it wouldnt go through wifi, and ran up almost $150, but T-mobile was kind enough to remove it off the bill. Just customer-friendliness, more than anything else.
The wife and I took a cruise a few years ago and I tried my damnedest to turn off anything even remotely resembling "yes, you can have a data connection". I wanted to leave my phone on in case something happened with the kids (and so I'd have a watch), I just didn't want it to DO anything other than allow incomming calls. Got home to a $200 phone bill because there was a checkbox somewhere I'd missed and cruise-ship mobile data is abhorrently expensive.
More recently, the Simple Choice plan they introduced last year which includes "free" international data roaming has ensured I stick with them for even longer. I travel quite a bit, so that + the wifi calling which works pretty much anywhere in the world has been a great thing.