At&t customer service error cost me my credit score

Hello - I don't know what to do and I am very stressed :( I bought a new phone through AT&T last march (pay in full) and was told I need to buy a new phone number to buy the phone (instead of transferring my old phone number). I found out later that this store was a third-party store and they couldn't force me to buy that new phone number. I only have it for a day, called customer service to cancel the line when I learnt the new, and transfered my old phone number over.

However, I have been receiving letters from At&t telling me I own X amount for that phone number. I called customer service several times to explain my situation and all of them told me I don't owe the company anything and shouldn't worry about the letters. Well, it keeps coming (higher each time) and I called them again in June. It turned out that I don't own anything to my current phone number (which I regularly pay) but I do have a payment for the other number that has been canceled months before. I explained to customer service that this payment was unfair because I do not know what it is for given that I canceled the line months ago and was reassured many times to not worry about the letters.

Customer service then told me they have canceled this payment and I have nothing to worry about. I confirm several times to make sure this information is correct. But today I received a letter from a debt collector company about this payment. I am very surprised and scared. I don't know what to do now. I am out of the States for the next 3 months so I am unable to make calls and this is my last resort.