Technically, after the audio/video call is initialized, the customer is charged in the first second, with the amount/minute set by the service provider and afterwards, at every 60 seconds.

Example:
Service provider Zoe charges $3 per minute:
When you call her, in the first second of the call (time 0:01) your account is debited with $3. 
after 60 seconds (time 1:00) your account is debited another $3.

  • if the total call time is 3:59 sec, the cost is 4 x $3 = $12
  • if the total call time is 4:00 sec, the cost is 5 x $3 = $15
  • if the total call time is 4:01 sec, the cost is 5 x $3 = $15
  • if the total call time is 4:23 sec, the cost is 5 x $3 = $15
  • if the total call time is 4:59 sec, the cost is 5 x $3 = $15