how to add calculated formula to a phone number field, when a 'Relate' field in the same module is chosen

I have 2 fields where one of them is a Relate field which consists of the records in the Contacts module. 

I have another field which is for a phone number

My aim is to auto-populate the phone number field when the Contact is chosen through the Relate field. 

how can i go about creating the formula?

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