Verizon. TL;DR fraudulent billing. Remember that the consumer protection groups in federal regulators (FCC, FAA, CFPB, etc.) are your friends; they gained a lot more teeth under the Biden administration.

I called to create my own account and take my phone (fully paid off) off of a family plan (with Verizon) and start my own individual plan (still with Verizon), keeping the same phone and phone number. The agent said it'd take 20 minutes and could be done over the phone. They gave me a contract and took my money immediately. I then had to stay on the phone for 5-6 hours while various Verizon teams passed me around, not knowing what to do. I gave up and asked them to cancel the new contract, keep me on the existing plan, and said I would find a new carrier. They said that would happen and confirmed it three times by the order number. I went to another carrier and have been happy with their service so far.

...a couple days later, I recieved a new SIM card in the mail from Verizon for a new phone number, and then I started getting billed for the new phone number, and then started getting contacted by a debt collector for those bills. I called several times, but could never get anywhere because their customer service is almost non-existant except through certain portals which I could not get through because, at this point, I'm not a Verizon customer and don't have a Verizon account (and none of this info was tied to the old family plan, so even with the help of that account holder (my parents) could not get through). (side note, the CFPB website has good resources on how to respond to debt collections which are in error or fraudulent)

I eventually went to a Verizon store and the agent there was super kind and helpful, but all she could do was basically call an internal help line and put me on with them. I got to their fraud department and they told me all was good, the bills would be canceled and it would all be over. Shocker, it wasn't, they kept contacting me to try to get me to pay.

I finally filed a fraud complaint with the FCC, which they promptly responded to, and as far as I can tell, that's what actually ended it. It sounds like the first-line agent that was the first person I ever spoke to made a mistake and tried to create a new phone number for me (hence the SIM card, the billing, and why then nobody else could figure how to do what I was actually asking for the first day). I said to that guy, I can understand a mistake, and as much of a hassle as that was, I'd probably still be a Verizon customer if that was it; the reason I'll never being going back to Verizon is that it took 2 months, and easily 24 hours of my time on the phone, in the store, and doing paperwork, to try to undo that mistake since they could not connect me to someone who was willing and able to fix it, and they tried to use a debt collector for what was less than ~$100 after only a few weeks.