Voip questions

Hello,

I’m new to Voip. I’ve just started learning about it.
I got some question I couldn’t find anybody to help me.

What’s the difference between Voip, Vofr, and Voatm?
What I understand now is Vofr and Voatm is layer 2 and Voip is layer 3. I’m not sure if it’s correct or not.

So, if we decide to use Voip and when we’re configuring, we do not need to care about the physical WAN technology. Because even if we’re using ATM, Frame relay or leased line as our WAN technology between our sites these will be layer 2 protocols and they will encapsulate the IP protocol anyway.
But if we decide to use Vofr or Voatm, the configuration will be different.
I’m not sure my english will confuse you or not. I need some advices about this. Could anyone please explain to me? Thank you very much.