Phoning is a waste of everyone's time. The admissions staff won't be able to tell him that. He'll find out in the very near future when he either gets a letter inviting him to interview, or doesn't.
Re: chances - cambridge do accept people with less than perfect grades. If you wrote a good PS, you may well get an interview. If you have a great interview, you may get an offer, B grade notwithstanding. You're at a disadvantage of course, but are given the opportunity to overcome and it and convince them anyway.