To make a phone number clickable and cause the user's smartphone to dial a phone number, select the number in Editor mode, click the link sign and enter the phone number according to this syntax: tel:+164612345678 (without spaces).
Say, you chose the Menu block ME301. In the Content panel, scroll down to the "More" tab and enter the phone number. Then highlight it, select the link icon in the text editor toolbar and add a link according to this syntax: tel:+164612345678. Website visitors on a mobile device can call the number with a single tap.
You can also use BF901 block from the "Form and button" category. There you can simply enter a phone number. It will automatically cause the user's smartphone to dial a phone number.