You say that calls are being dropped at the central location. If you have a hub and spoke setup wherein the hub is the 'central location' and the remote locations are using the trunks from the central location for inbound and outbound calls, I would start looking for the cause at the central location. What type of trunks do you have? What do you experience when the call is dropped? e.g. fast busy, clicking, etc. What does the display read when the call is dropped? Does it go immediately back to displaying the date and time?