AT&T cell phone bill and change of area code

When we moved to the other area, the cell phone signal was bad and the customer service suggested a change of area code. However, after the area code was changed, we got the new account number (without notice) and AT&T charge about $300 contract early termination fee on our previous account. Actually we just renew our contract a couple of months ago and AT&T claims when we changed the area code, then we started the new contract. AT&T also charged some other fees around $100.

So, before you try to change your area code, make sure these things won't happen. Now I have contacted custer services many times and it seems AT&T doesn't want to solve this issue.

What can I do? I appreciate any suggestions.