You would need to get a US number from a SIP/VOIP provider, then just have the restaurant forward their number to the number you registered. two things though, you will get ALL of their calls using this method, so the restaurant would essentially not have a way to take calls unless they install a second line or get a second DID. Then, as others have mentioned, the call quality will probably be crap.

I wouldn’t recommend it. If cabling is a problem, you might want to consider WiFi phones, then you would just have a thin power cable with a wall wart.

The description your user gave was one phone calling another phone, but then you described it as “intercom/walkie-talkie/phone to phone on the LAN” which isn’t the same thing, so I’m a little confused. If they just want to be able to make calls from phone to phone, you can either use direct IP calling or get a cheap PBX, the intercom/walkie-talkie would be called paging, which is something else entirely and not usually done through phones but through paging interfaces.