Skype SIP trunk cannot accept 3 simultaneous calls

I installed AsteriskNow then bought a Skype number and I configured successfully. Now I have 3 channels, in theory, my asterisk can receive 3 simultaneous calls at a given time. But the problem is: If I have 3 phones and I click call buttons on 3 phones at the same time (or almost that), one of them will receive ‘line busy’ message and 2 others get connected to my asterisk. But if I click call buttons on 2 phones at the same time, after 2 these phones get connected to my asterisk server, I click call button on the third phone, it also gets connected. This means that, at that time my asterisk is receiving 3 simultaneous calls. So, what happened?