HTML telephone links follow the same structure as links to other website pages, except the website address is replaced by tel: and the number.
For UK domestic telephone numbers, the link would be like:
<a href="tel:0118-496-0400">0118 496 0400</a>
Here I have used one of the unassigned TV and drama numbers as set aside by Ofcom.
To convert this to an international number the zero is replaced by a plus sign and the country code. For the UK this becomes +44:
<a href="tel:+44-118-496-0400">+44 (0)118 496 0400</a>
Dashes are used as a replacement for the spaces in the number. It is visually easier to see a number divided by spaces or dashes compared to a long number. If you wish these can be omitted when creating the link.
<a href="tel:+441184960400">+44 (0)118 496 0400</a>
