AT Z S46=0 &Q0 &D2
AT Z H0

Line 1 is the initialization string, which needs to set the modem to
9600 baud, no compression, and no error correction protocol.

Line 2 is the shutdown string, which should hang up the line (if dropping
DTR hasn't already) and reset any parameters you like.

Settings known to work:

Practical Peripherals 14400FX V.32bis
AT Z S46=0 &Q0 &D2
AT Z H0
