Pulse dialing detection for multiple telephone sets can be done using a timer interrupt.
The basic idea is to start a timer at 10 msec.In the ISR, read the status of the OHK pins for each set.If a pin is found high, set some variable to confirm (in the next interrupt) whether it remains high for another 50 msec at least.If yes, observe if it goes low for 40 msec?

You will have to use multiple flags, and device some scheme as stated above for necessary decisions.

And I am sure, you will need an ICE since this can't be done on simulators.