Dr P,
To put it in a nutshell:
1. How many seconds will a customer save per billed call (on average) if he is billed in 6-second increments, instead of being billed in 60-second increments?
2. How many seconds will a customer save per billed call (on average) if he is billed in 6-second increments with a 30-second per call minimum, instead of being billed in 60-second increments?
3. How many seconds will a customer save per billed call (on average) if he is billed in 1-second increments instead of being billed in 60-second increments?