I've looked at a similar situation, and at first glance the mobile extension sounds like a winner, but turns out it's not exactly what you want. Look at Multi Line Client, or MLC. It's an app that makes your cell phone function as a station. Complete with programmable keys. Works good on wi-fi within the company network. Supposed to also switch to cell mode off wi-fi, but I know there were issues with that earlier. My customer doesn't use it off site.

If you end up with MLC, just a heads up: only activate the app on each cell phone _once_ . If something doesn't work you may be tempted to activate again. This just uses up another license and is a pita and time consuming to get it cleared.

Jim