How long can this phone bill stay on my credit report, the original balance is from 1999?

The original creditor was ATT and now the creditor is CSC Credit Solutions Corp. (the same company that owns my credit file). I have disputed the amount with the credit reporting agency because the balance is incorrect ($700 more)and because the bill is older than 8 years. After filing the dispute CSC calls me the same day and says that the amount has been verified and that I need to pay them that same day.I'm disabled and being harred constantly by CSC and then today I find out that CSC and another collection agency is trying to collect on this same debt. Also, I have another phone bill that shows on my credit report that has been charged off, the collection agency wanted me to pay this bill too. I'm not sure what charged off means. I appreciate any advice on this. I'm trying to repair my credit.