Some options you can add to your /etc/sysconfig/network-scripts/ifcfg-ppp*
Dial the connection when it is needed:
Disconnect after a specified period of inactivity
IDLETIMEOUT=<number of seconds>
Automatically reconnect on disconnect
Wait for specified number od seconds before trying to reconnect after being disconneted.
DISCONNECTTIMEOUT=<number of seconds>
Wait for the specified number of seconds before trying to reconnect after a failed connection attempt.
RETRYTIMEOUT=<number of seconds>
That should get you started. Replace the * in ifcfg-ppp* with the correct number. Probably 0.