Should I offer to come to campus if given a phone interview?
It is worth asking and is probably worth a visit even if they would rather stick to a Skype interview. They may prefer that so as to treat all candidates equally, but if that isn't necessary, they might prefer you to come in person. 

It is a good way to get a feel of a place and to meet a few students and several faculty. I don't see any downside in asking. 

"I'm actually quite close and could easily arrange a face to face visit. Would that be preferable?"