How to insert phone No in a field according to the selected client

I have two custom fields in a custom form. One field is to select a customer and the other is to insert the phone number of that same customer.

The field to obtain the client is of the type Link, which links to the Customer doctype.
How could I obtain the telephone number, in the case it was established, of a specific client?

This video can help you.

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file doesn’t exist

Could you please explain here the method, or reupload the video? because it is not available in the link.