Where are you located? Their may be some Engineering Students around who would be happy to help with your project.

Each number caused a different number of pulses (1 + the number dialed) to be sent on the line.

Each "pulse" was the equivalent of quickly hanging up the phone (removing it’s load from the line) and then picking up the handset again — the phone line consists of a power supply of about 20 volts (but you could use almost anything in this application, since you are not really talking to the telephone switchbank gear) connected through a slight resistance, so that the line voltage drops when the phone is off-hook.

Here is more information on how that worked:
http://en.wikipedia.org/wiki/Pulse_dialing

Counting these pulses with a BASIC Stamp processor, and triggering a Quadravox MP3 player chip (for example, the QV600) in response, would do what you want.
http://www.parallax.com/detail.asp?product_id=27207
http://www.tetraphon.com/

You can know the phone has been picked up when the line voltage drops for a long period (longer than say half a second). You can differentiate this from dialing pulses, because they are each very short (a tenth of a second approximately) and regularly spaced. So the software would play a dial tone through the speaker when it detected an off-hook phone, then would play a short phone ringing sound file after detecting pulses (you want to wait a little bit, half a second or so, after the final pulse to be sure no more are coming), and then play the interview file based on the number dialed. It would be simple enough to detect a re-dial during the interview as well, and re-trigger the Quadravox chip to play the new sequence of files.

Email me for more info, I can design such a thing easily if needed or send more detail to help one of your people do it.

the simple solution is no dialing just pick up the phone for a connection - the existing switch would trigger the recording using the remote plug in the player - new speaker and you’re in business

hope you get a better solution !
AND YOU DID ! at least the first answer - the second isn’t