Secure voice software issues

so I haven't been able to verify which V.42/V.42bis/MNP options will let you squeeze out the start/stop bits to let you fit 13.3 kbps of data over it,

Whenever V.42 error correction (LAPM) is enabled, synchronous HDLC frames are what actually pass over the link. So the start/stop bits are already removed. Unfortunately, the packetizing done by LAPM adds delay we don't want for a real time voice application. And if you turn off LAPM, you return to sending the start/stop bits over the wire. Most V.32 and V.32bis modems provide for direct synchronous operation, which would let us have our cake and eat it too, except that few PCs can speak synchronously to a modem. This may require some extra hardware (sigh). Phil