Facetime Question

[Dimwit]

I'm wondering, Does the wifi network the iPhones are connected to need to be connected to the internet IF the iphones are on the same LAN?

I am thinking about this because as an industrial electrician, when I am on a machine breakdown, it would be nice to be able to communicate with someone as well as send them realtime pictures. Some of the places I have to get into to do work are not conducive to multiple human bodies. Many companies do not want to provide internet access, but, might be able to be sold on providing a standalone network for something like this.