1. In the text editor type in your phone number

2. Highlight it and click on the insert/Edit link button in the text editor
3. Select the option for "Existing Page" and put in that field e.g tel:2318433330
Note it will prompt you with the following

"This specified url was not found.

Click Ok to continue anyway or
Click Cancel to enter another url."

Just click OK and you are all set.