The Facetime make me headache for last 2 days. I try to test Facetime among iPhone4, iPad2 and Macbook Air. All are not success. After I try to ask help from my mobile operator (dtac), I got some hopes and ideas to test (the call center was very helpful and kindness).
What I did :-
1. I try test Facetime outside my home, at Starbucks which I have TrueWifi account. I success in do Facetime between iPhone4 and iPad2 outside home. When I come back home and test again, it keep working.
2. The problem now is on Macbook Air only. First I uninstall the free version of Facetime and buy the upgraded version from Mac Appstore (for Macbook not iPhone) with $0.99 . From installing it, make me need to go through the sign-in process again and found out that I can’t sign in with any of my Apple ID account, weird? So, I try Google and found the useful article that talking about this. What I need to do is to go to Macbook System Preferences > Network and Add DNS for the current connection I used with 18.104.22.168 and 22.214.171.124 . After hit Apply button and turn off and on both Wifi and Facetime app. I can sign in it successfully. And I can success do Facetime from Macbook to iPhone4 and from Macbook to iPad2.
So, if you have problem like me on Macbook Facetime, try do above step.
Thanks to Paralaptop.com (http://www.paralaptop.com/macbook/facetime-for-mac-not-sign-issue-fix.html).