If you use your smart-phone (iPhone or Android) for surfing on internet you will have notice that some times is very frustrating or very difficult to copy and past a phone number in order to use it for a phone call.
A simple and quick solution for all the web designers/developers to avoid to make the user s life difficult is to set the telephone number as a link.
Please call <a href="tel:+441234567890">(+44) 0123 456 7890</a>
instead of
Please call (+44) 0123 456 7890
A link like that will not have any side effect for a normal desktop or laptop user but it will be very helpful users that use a smart-phone or a tablet.
So a good practice is to always use this telephone number link
<a href="tel:+441234567890">0123 456 7890</a>
tel: is also compatible with some telephone softwares.