Jill's voice is definitely not Claudia. The Joyce Gordon answer is the most widely accepted.I once read that in her early Italian movies, Claudia was regularly dubbed by another Italian actress. I believe there was some resistance to her husky voice.I once saw a documentary that claimed it was common practice in Italy to film an actor or actress in a movie, and then use a different actor or actress to dub them in the Italian Language version. This sorted the problem of good-looking actors/actresses who didn't have good voices.
Plenty of movies with Claudia speaking English in her own voice (The Pink Panther, The Professionals). Very husky and heavily accented, neither of which applies to Cardinale's dubbing voice in OUATITW. It's strange that they didn't get her to dub her own voice though.
It's strange that they didn't get her to dub her own voice though.