Remember that if you have an active Czech SIM card in your phone, that friends and family can call you from abroad and it won't cost you anything (assuming of course that you don't have SKYPE - a preferable method as it is free then for both parties).
The international telephone code for the Czech Republic is:
00420 or, +420.
To call to the Czech Republic from abroad, and I'll give my number here, you should call:
00420 775071550 (or +420 775071550).
If calling from Canada or the USA:
011 420 775071550.
Hezky den!/Lovely day!