Connect via telnet or ssh.
At the unit's prompt issue a "/c modem".
This will connect you directly to the units internal modem. By default echo is disabled.
The following command will enable echo so you can see what your typing:
After issuing the command the modem should return an "OK" indicating it has accepted the command and echo has been enabled.
To dial out type:
Where "X" is the phone number you wish to dial.