Have you looked at the pulse output of these telephones? Pulse dialing is usually used when the central exchange runs on the older mechanical stepper switches and relays. The pulsing mechanism on a lot of these phones is nothing more than a relay that opens and closes the number of times indicated by the button pressed. It is very possible that these pulses are full of contact bounce which will play havoc with any counting scheme you come up with. If you have a scope make sure you have nice, clean pulses. If bounce is present you will need to implement some type of debouncing scheme.

Hal