Cellc is a disgrace

This is the worst, most *********** and most ********* and unprofessional company I have ever dealt with. I have paid up my 2 cellc contact, however one other balance is on my credit record with the acc number ending in 18, with a balance of 1800. I am completely unaware of where this comes from and not one person working cellc is able to assist me with this. I have attended all avenues on trying to get it sorted, they keep saying they cannot access the account. Extremely disgusted.