Making local calls from a real estate 'open house'

Inspired by this thread here
[if I did that right]
Personally, I think the question of legaliity was answered in that thread…but

I was trying to construct analogies for using a neighbors wireless internet access.
That’s when I came up with:
If there was a real estate open house, could someone walk in and just use the phone to make local calls?

Other than being incredibly tacky, is -that- illegal? And it strikes me as unethical, but I’m not sure exactly how. Is this just so much beyond the pale that they haven’t written a law against it?

Mods, feel free to move this anywhere.