title: Different action on hyperlink depending on whether device is able to make phone calls
body: <p>I'm building a mobile site with a "call" link in the header.  I'd like to be able to differentiate between mobile devices which are capable of making phone calls (ie. phones) and ones that don't (ie many tablets that don't have SIM cards/VoIP software) in terms of functionality - when the user taps the link, I'd like it to behave as a tel: protocol link if it is capable, but if not, to route the user to a "contact us" page.  Is that actually possible?</p>