I had a pre-paid phone for the longest time (in the states). I kept pre-paying it even after I moved to Japan because I needed the SMS codes that were occasionally sent to my number. This was with T-mobile.

One day T-mobile just took my number and said you no longer have this number. No warning, nothing. Talk about a great way to get any non-2FA accounts hijacked... just gave the number to someone else. This was despite me having frequently re-filling the balance and using the number every week.

Is this crazy or what... I even checked to see if maybe they discontinued the pre-paid program at that time, but nope, it was still up and running (no idea if it is now).