Your modem connection should be established automatically. Type
plog to check the login sequence with your ISP. If
there is no output check the logs in /var/log/ to get
a clue on potential problems.
If you want to be sure that your modem and/or script parameters are OK before backing up ppp.lrp, you can launch the connection manually just by typing pon. Use the plog command to see how the connection is going and poff to close down your ppp connection.