In my opinion "face-to-face talk"means that when you talk to other people you two are sitting together(in a house) But "chat via online voice"means that you two arenot in one place.You comunicate with others via internet not in person.

When you talk face-to-face in a language that you are learning, is very useful seeing the lips of your partner to understand better. In the online chat, you only have the sound, and is a litle bit dificult sometimes guess correctly what your partner is saying. Cheers