If I remember correctly the M700 had a telephone style handset as an option. That means everything you need is at the microphone jack. If you can track down a spare connector you build a cable for you modem and swap back and forth. The downside is setting the volume at the right level every time.
Yes, Dave is correct: the M700 was designed with a handset option, so there's audio at the mic connection.
Here are the pinouts:
1. AF Audio from main board
2. PTT PTT pin. Transmits when shorted to Pin 3
3. PTTE Ground pin for the PTT
4. MIC Ground Pin
5. MIC MIC Pin
7. AF Connected to the built-in speaker. When the speaker switch is turned on while pins 1 and 7 are shorted, the built-in speaker is used