I asked Verizon to remove two devices off of my account almost 10 years ago. Instead of removing them entirely, they just made them suspended and kept charging me for their use. I'll give them one chance to repay me the money they have overcharged me for. After that, I will contact a lawyer.
Lawyer: You asked Verizon 10 yrs ago to remove these 2 lines? You didn't check your bill to confirm they were removed and just paid the bill for these 2 lines for 10 yrs(120 billling cycles)?
Lawyer: (thinking to themself... They paid Verizon a fairly large amount needlessly, I should be able to get quite a lot from this person) Of course I'll take your case. Let me tell you about my retainer fee...
Keep in mind, your customer agreement states you have 180 days from receipt of your bill to dispute charges. That wipes out 9.5 years of your "overpayment".
The amount you will have to pay a lawyer would likely be FAR greater than any amount, if any, you would be able to recoup from Verizon.