It isn't the phone conversations alone that prove the guilt of the twins, it is also Columbo's demonstration that the crime could not have been committed by one person alone. So if there were two people involved in the crime, the only "pair" of suspects that Columbo has are the twins. Still only circumstantial I suppose, but a bit more solid than some of the other circumstantial endings.

I disagree with you who think it is a weak episode. Martin Landau is superb and I watch it quite often.

In that last scene, I always liked the way that, not only Columbo, but one of the two brothers, felt the need to let Mrs. Peck down gently.