How can I initiate a modem connection from the command line?
To connect a network device, use the System Events dictionary:

tell application "System Events"
    tell current location of network preferences
        connect service "Sprint 3G"
    end tell
end tell


assuming the name of the service is Sprint 3G:



To disconnect, change connect to disconnect:

tell application "System Events"
    tell current location of network preferences
        disconnect service "Sprint 3G"
    end tell
end tell


Once you've saved your scripts with names, you can use the terminal open command to kick off either script.