I need my phone to be on Wifi/4g to use Speech to Text?

S Voice, just like Google Now, is a cloud service. So, one way or another, it needs to access the internet.
 
According to Samsung, the S Voice engine in the Gear S is independent and doesn't require cloud access.

At 58:10...

 
Weird, cause I was trying to use it the other day, while connected to my phone mind you, and it kept saying network error.