Is it acceptable to call my supervisor at home? He almost never answers the phone during office hours.

To be honest I think that would be a bit weird and intrusive, but then I only ever contact my supervisors by email or office phone in an emergency (have done that once when all my trains were cancelled/delayed and I was running late for a supervision). But then if your supervisor has given you his home number then surely you have permission to use it. I would only use it if he has given it to you personally, as opposed to if you have found it on a university database or something. But I think you should email first if that is how you usually communicate and only call him if it is actually urgent.

She has given it to me personally, when she invited me to her house for dinner - ages ago; but I still hesitate to call her, although my matter (some references I have asked for) is a bit urgent.