Your formula field typehere is text soan example to update contact phone number when you must returnassign the account to a text if yourcontact.
Step1: create Workflow on contact.Add condition matchesto execute the workflow. Something like thisif account is not null.
if(ISBLANK( Customer_ID__c ), 'text if it is empty', 'text if it is not empty')
step 2: Update contact Phone as Account phone.
Your formula field type is text so you must return a text if your condition matches. Something like this
if(ISBLANK( Customer_ID__c ), 'text if it is empty', 'text if it is not empty')
here is an example to update contact phone number when you assign the account to a contact.
Step1: create Workflow on contact.Add condition to execute the workflow. if account is not null.
step 2: Update contact Phone as Account phone.