night hawk extender or orbi

If your internet phone device just plugs directly into a router and works, it'll work with orbi.  If it needs something special (not sure what) then its a bit of a gray area. You say its his work phone? I'd contact the company that provides it to ask them as they'd be the experts on that specific device and not Netgear. 

I've been using orbi since it came out and like how seamless it works. You don't have to worry about different ssid's, it automatically roams, dynamically switches between radio's and devices.  Basically it just works. (for me anyway, some have had issues)  I've used both wireless extenders and mesh networks and I keep going back to the Orbi setup.