The ADT alarm system at my company required a few phone lines. While it's best to use REAL phone lines because they come with a battery and keep you secure during a power outage, this is an interesting stopgap. It doesn't make a modem connection, instead is dials a number and after connecting, dials a feature code to the reciepient toll free number. It dials these codes quickly and so the configuration must use DTMF of inband.
Echo cancelation OFF
DTMF = inband
use ulaw ONLY
Use preferred codec only
I believe that DTMF is the trick that really makes it work.