Systeme io Questions and Answers (FAQ)
In the hyperlink section: enter your phone number by adding tel pre-text. It is recommended to add a phone number with a proper format that includes the country code, although it may work even without the country code if you are calling within the same country.
Format
tel:+(country code)(area code)(phone number)
Example
tel:+60123456789 (with country code)

The hyperlink is now clickable and on the phone, it will automatically open up the phone dialler.