You won't find it until you're finished with Penelope Haines.
When I lose my cell phone around the house, I call the number and listen for the ring. (I actually found my mother's phone in her garbage can once by doing this, in fact.)
Jeremy's notebook says he lost it after someone in this case tried to scam him. When you meet a scam artist, try calling Jeremy's number.
That'll be outside of Lisa Tenzin's house.