Facetime with andriod tablet

Is there any app available in appstore to call an andriod table (just like facetime, with out any login or creating account).

I am planning to buy andriod (cheaper) tablet for my mother, so that I can video call her from my iPad. My mother don't know anything about computers or tablets. I am looking for a simple app like facetime so that she can just click and call my iPad.

please suggest.