I am interested in making phone number links to begin phone calls for users browsing a mobile site.
What are the differences between the tel and wtai url schemes? Are there other schemes?
How should they be used and in what circumstancs?
Do some devices only recognise one scheme or the other, is either scheme deprecated or likely to be?
AFIK WAPI WTAI spec belongs to wap protocol and if phone supports WAP/WML it would support WTAI.
Tel Uri would be the most common and Apple's tel meta tags only recognised by iOS devices.
If you have time and resources to spend on it you could have a look at http://wurfl.sourceforge.net/ to get device capabilities. If not I would use tel Uri
