In most cases, when something starts working incorrectly, it's a result of a change. It might be hardware (something on the network, new bridge?), or a firmware/software update. It might also be software corruption, but that usually creates a multitude of problems.
Were there any changes made at the time the phones began dropping calls?
Since it is all phones, it is either something on the network (hardware), an update to the 3CX server software or hardware, or an update to all of the sets (firmware/options).