How do I fix a bill that went to collections and now the company doesn't exist?

When I was younger, I was gullible enough to fall into the cellphone carrier HELIO's trap. I was 17 and put 2 phones under my name. I returned the phone after 2 weeks and the guarantee was 30 days. I walked up to the sales rep with both phone in the boxes untouched just like I got them. I asked him if i owe anything and he said no he would take care of it and I don't owe a thing. He said if a bill comes in the mail to just ignore it because it would be the activation fee and such.. so i got it and ignored it along with the other yrs. worth i tried calling and they said i used it for ONE minute outside of it 30day guarantee so i owe . This is affecting my credit score severely! all because the ociate didn't cancel my account. what do i do?