Depends. If it's a quick 10 minute job it'd probably cost more logging the call and invoicing. If it's turning into a half hour consultancy then I'd charge at my hourly rate.
As a rule though, I *never* give out my mobile number - I value my non-work time too much for that. It also sets a precedent that you can be contacted and issues solved instantly - not desirable from a time planning point of view.