It would help a little to understand your problem if you could tell me the maximum number of digits sent by the telephone. Will it always be one digit or two digits, or will it be variable, sometimes one, other times two or more? Also, are the phones push button providing pulse output or are these the old, rotary dial units?
Per your specification, the pulse stream for any one digit will arrive at a rate of 10 pulses per second, with the number of pulses representing the actual number, where 10 pulses equals the digit 0. If more digits are to follow each group of pulses is separated by a minimum of 800 ms.
When the first pulse arrives could you not start a timer which will then subsequently be stopped by the next pulse, the timer value saved and the timer reset and restarted. The saved value is compared against a range of values that will be either close to 0.1 second or 0.8 seconds to determine if the previous inter-pulse gap just measured is either part of the current digit or the indicator of a new digit. If the timer value greatly exceeds 0.8 seconds then allow for a certain amount of time to elapse to determine if dialing is finished and the user has just simply failed to hang the darn thing back up. If ON-HOOK is signified by the line going low then you will need to also measure each pulse for a possible on-hook situation.
In a nutshell, then, you will need to measure the length of each low pulse for a possible hang-up, and also measure the gaps between low pulses for either 0.1 or 0.8 seconds while allowing for the inevitable situation where the user fails to hang the phone up.

Hal