We're frequently changing the number of calls we offer to users, so this is subject to change at any time. We change the number of free calls since the costs for calling are subject to change, and it's heavily dependent on the location of the call.
We calculate the number of free calls someone receives based on where they are in the world. For instance, in the US, users get 3 calls a day. A day is calculated every 24 hours; meaning: if you sent one call at 2PM and another at 4PM, by 2PM the next day you'd get one free call, by 4PM, you'd get another, so by 4PM you'd have two!
Currently, the allocation of free calls is:
US - 3
Canada - 2
UK - 1
Spain - 1
Australia - 0
Italy - 1