The only restriction is that the free calls are time-based, you can check the free time durations of different countries here. US and Canada have the highest free-time limit of 4:05 and 3:32 respectively. On reaching the free time limit, your call would end and you will see this message.
Their is also a paid-service offered on registration which allows you to add funds to your EVAPHONE account and make calls.Check out the call rates for different countries under the paid service here.