There were several weak links in this episode. The fresh water on the deck not evaporating; the weak 'gotcha'. When Hanlon claims he called Eric twice from the box, Columbo says, "Unfortunately, the telephone company's records can't prove that." So, Columbo did look into it. I don't know the exact methodology of checking phone records in Los Angeles back in 1972, but I think I've seen an article about this somewhere. For the purposes of the Columboverse, let's assume he did check them, as was indicated in the show. If that's true, then, as Hanlon said, "That's not MY problem."