Not necessarily. Check out the Valcom IP paging. https://www.valcom.com/default.htm
The Valcom support people should give you some good ideas.
You would have to check what the warehouse will have for a network and what they will have for internet at that warehouse. If you get involved you can set it up right.
I don't have any first hand experience on this set-up myself. However, I did some research on it about two years ago, and according to the Valcom people it works pretty slick.
When your customer starts putting more phones in---then they can upgrade to another 6x.