I'm not clear about your follow-up info...but there is no SOL on taxes. If that is what the cell company is tying to collect, pay it right now.
If the amount owed is for service, then read on...
Traditional cell phone accounts are written - the contract you sign specifies a time, etc. The SOL in Utah for a written account debt is 6 year.
SOL refers to the amount of time a creditor has to take legal action. It does not mean that a legitimate debt isn't owed after the 6 years is up.
If you do not think this debt is legit, then you need to take the proper steps. Other than asking for a mailing address, all communication should be via postal mail, using signature proof of delivery.
Send a debt-validation request:
http://consumerlawyer.mn/wp-content/uploads/debt-validati...
(Don't worry about the state this link is from, it applies nationally.)
If they are able to validate, then pay the bill because you do owe it. If not, then you can send a no-contact letter stating they are not to contact you again, except in the case of a lawsuit. Which may still happen, as they have 4 years, and then you'll need to go argue it there. If they can't validate, you'll win, but you'll still have to show up to court and waste time on it.
Keep all documentation you have in regards to this account forever. I mean literally forever. If it gets sold to another collector later on, you'll have to do the letters again.
Added: If it does pop up on your credit report, and it is valid, it will remain there for about 7.5 years. If it is not valid, then you can dispute it with the reporting agency.