Should I ask for a confirmation of my interview if I answered late to the invitation?
You should attempt to confirm the interview.  It's quite possible that somehow your email was somehow missed or there was a miscommunication.  The people doing this stuff are human and make mistakes.  Even if they are actually planning to do it, getting confirmation is a reasonable request.  If you don't hear anything, you should probably be ready by the phone at the appointed time, but don't be surprised if they don't call.