What is the proper, universal format for creating a clickable hyperlink for users on mobile devices to call a phone number?
Area code with dashes
<a href="https://stackoverflow.com/questions/13662175/tel:555-555-1212">555-555-1212</a>
Area code with no dashes
<a href="https://stackoverflow.com/questions/13662175/tel:5555551212">555-555-1212</a>
Area code with dashes and 1
<a href="https://stackoverflow.com/questions/13662175/tel:1-555-555-1212">555-555-1212</a>
Area code with no dashes and 1
<a href="https://stackoverflow.com/questions/13662175/tel:15555551212">555-555-1212</a>
Area code with dashes, 1 and + sign
<a href="https://stackoverflow.com/questions/13662175/tel:+1-555-555-1212">555-555-1212</a>
Area code with no dashes, 1 and + sign
<a href="https://stackoverflow.com/questions/13662175/tel:+15555551212">555-555-1212</a>
