From VoIP.ms Wiki
VOIP.MS charges their calls based on a 6 seconds period increment. This applies for all countries except for Mexico where it applies a 60 second increment.
Here is a short explanation about how this works.
The Billing Increment is a call duration measurement unit expressed in seconds. 1 minute is divided into 10 periods of 6 seconds.
Let's say there is a rate of $0.50 USD per minute, then the user will be charged for 5 cents per each period of 6 seconds.
(0.05 USD )* (10 periods on a minute) = $0.50 USD per minute
10 periods of $0.05 in a minute
The system always registers the number of cycles of 6 seconds, except for Mexico, therefore if you make a call of 27 sec, you will be charged for 30 seconds as you are running in the 5th period of 6.