Still receiving invoices for service that we tried to cancel in February

AT&T is still invoicing me for phone, internet and cell service, but we have cancelled our AT&T service.  I've been calling and writing since February to cancel this whole business plan and I have a folder full of documentation of who and when I called.

In January we received an unwanted cell phone, and since February I've tried to get out of this whole package.  Now I'm receiving threats that you'll turn me over to a collection agency.  If AT&T does not resolve this, I will be calling the Better Business Bureau, as you are threatening my credit at this point.