Thinking about wall mounted intercoms. In the modern world, that sounds terribly inconvenient. Working on something, and a BEEP demanding you stop work and run over to the intercom.
One job I worked at had push to talk Nextel phones. It seemed a bit strange when one could simply call on the cell phone, but in fact it was fairly convenient.
It looks like Nextel is long gone, but there are other push to talk phone solutions available, although primarily for businesses. I'm not sure why they don't have family plans, or perhaps it is an option.
As far as the problem with cell reception inside the shop. Many cell services will jump onto WIFI. So, if you get a good WIFI signal in the shop, then your phone should work.