Something does not add up.If you have a 3640 with a T1 at your office, what do you need something so basic as the ATA to handle the calls on your location???
It may be that you are using the 3640 as a gatekeeper, but that will not need the T1 card on the 3640.
As a good start, look into the ATA 186 mneu to see where you are pointing those 186 .Look for the field named “gatekeeper” and “gateway”.
If you have static I.P. addresses on those remote locations, with some cheap gateways you could make a good size VOIP network, with multiple remotes.
If you have any questions give a holler