Issue with Wireless account on Iphone-6 Next Plan

I am not sure if this is the right place to post some queries rather concerns .

I have heard that att has a healthy ratio of atleast 90:10 in terms of positive and negative feedbacks, but unfortunately I am among the 10 people who are unsatisfied with the services extended .

Almost 3 months back I baught a new Iphone-6 on ATT next plan , but the phone came with a new contract postpaid number and I wanted to use my existing ATT perpaid number . After communicating my requirement to ATT store they suspended the contract number and added my prepaid number . But only when i recieved the next bill I realized I am charged for two connections (which was never explicitly mentioned by the store representative) .

After talking to customer care they admited this as an issue from their end and advised me to talk with store to fix the issue . The store manager iagreed that they would correct this issue and remove one line and provide me the credits . I am not sure what they did (they never told me too even when i asked) but after almost 4 hours of waiting they told me issue is sorted out and when i checked on my account of 593 $ was added which the store claimed will be credited back to me in a month's time .

Since then till date no credit has happened and I have been following regularly with customer service and store when they keep saying they are working on it .

They have also told me to pay the bills only when crediting has happened , so I am still waiting and I am dues have almost crossed 850 $ (for 3 months usage - unbelievable and illogical) .

Hopefully someone can advise me here on next steps