How do I include extensions in the tel: URI?

Question

I currently have a webpage serving up phone numbers, some of these phone numbers have extensions so I have written the HTML like this:

Answer 1

Standards at thenewcode from 3 months ago suggest using a microdata pause.

Related: Different standards persist across different external platforms and may change the processing of URIs. Click to call features on Google Developers docs do not specify

Example: Office's Skype uses x to represent extension within skype.