I would guess it uses a Z80 core with integrated peripherals - lower power than any x86 at the time, and pre ARM. Probably made by Sharp, as they did do a version of their Z80 core for copiers, and these had a pretty big number of IO pins, and bank switching built in.
As to the isolation board, probably a telco who wanted to keep any competition away, by having a near impossible to do spec for other equipment, while their own had a much more relaxed spec.