I don't know if this works for every phone service, but it works for mine:
Leave the phone off the hook for a long time (10 minutes?). After the
recorded message spiel ( ...hang up, if you need help....several minutes
of loud screetching tones...) you get dead air. From there, I am
able to connect modem to modem using "reverse answer mode". This
modem is invoked on most modems by issuing the ATX command on both machines.
I'm told not all modems support reverse answer.
Good luck,
Ray Schuler