The problem is likely that the location where these phones are really at is near the WiFi router that used to be at this address. No amount of messing with their router will help with fixing this, since those phones aren't there to begin with. The couple might have better luck hitting up the previous tenants/owners.

Based on this line from the article, I'm almost certain that this is the real answer:

> It started the first month that Christina Lee and Michael Saba started living together.