The set amount which you pay for the first 30 seconds of your call.

Flagfall is the initial amount charged when a call is first connected, you could also describe it as a call connection charge.

A rather peculiar word that has nothing to do with neither flagging nor falling, but is what a service provider will charge you as the initial fee for any call you make, regardless of duration. The fee may vary by the type of call you make or who you make the call to.