Go to desk.zoho.com.

Find the person by using the email, name, or if you have a ticket for that person that is opened/closed, go to the Zoho Desk ticket.

Once you have the ticket open, click on the name as circled in the image.

Click on either option of phones and you will be able to edit the phone number.

Click on save.

You will have now the option to call that person (as long as you are online to make a call).

To make a call on Zoho, refer to this Screensteps article:

https://discoverhealing.screenstepslive.com/m/70653/l/1604403-how-to-make-a-call-on-zoho-desk