For voice calls, your billing begins when the call is answered. It ends when the call is hung up. You don't pay for ringing time with Tropo.
For text message sessions, you are billed only for the message, and not for any time your application is running.
Tropo allows your Scripting session to run before and after any calls or text messages, to allow your application time to gather configuration, clean up, or send reporting back to your back end. You are not charged for this extra application session time.
Making calls that you know will not complete - for example by making a call and immediately hanging up once it starts ringing, or running a Tropo application without making calls or sending SMS - is disallowed by our terms of service. Applications that consistently do this or otherwise abuse non-billed application session time will be blocked as abuse or service.