Corporate voice phone bill from AT&T

From: Ed Amame, Chief Accountant
To: You
Re: Can you hear me now?
Even though we are running T1 and T3 lines among all of our offices and stores, our corporate voice phone bill from AT&T is in low six figures a month. Is AT&T providing us with something we can’t do ourselves? Are there lower cost IP Telephony (VOIP) options for us to consider?
Are there considerations we need to factor in other than price? Can you get me a one pager on this? (NOTE: I don’t want to change the WAN configuration, just the voice phone bill.)