Two Tasks Needed
1) In order not to update on boot, go to System --> Administration --> Network; select the modem or dialup connection; select properties, and uncheck "Enable this connection". This should prevent auto-dialing when you boot.
2) Now, you will want an easier way to dial on demand. To make a desktop icon for your dial-up connection, go to Applications --> Internet --> GNOME-PPP (assuming you are using that); right click on that; and select add launcher to desktop.
You should be able to dial on demand.