Your "Monthly Statement" is likely at least partially tied to your PRI's... "Digital Loop Service" is T-1/PRI circuits.
I would reach out to AT&T Business, find out who your rep is, and have a sit down. We see this from time to time, but not as much as we used to... PRI circuits used to be crazy expensive, but now days you can get them with unlimited calling delivered via SIP to a gateway for 1/10th the price of old, traditional PRI's... And don't just resign a contract with AT&T (or any carrier) without getting competitive bids.