About all you can do is dial it from another phone and walk around and listen for its ring. In the house, in the car (I found my wife's once that way) or in the last few places you were before you noticed it missing. Even in public places if it mat have fallen between cushions in chair, booths, etc.
Additionally, if someone answers it you'll know not to look any further if they are not willing to return it to you. I would advise against meeting them someplace to recover it unless it is a very public place with many people around.