If you need to format a phone number field in an Entity Form in CRM, use the following JavaScript code to accomplish this:

    function formatFieldPhoneNumber(executionContext, fieldName) {
        formContext = executionContext.getFormContext();
        var value = formContext.getAttribute(fieldName).getValue();
        var phoneFormat = formatPhoneNumber(value);
     
        formContext.getAttribute(fieldName).setValue(phoneFormat);
    }
     
    let formatPhoneNumber = (str) => {
        //Filter only numbers from the input
        let cleaned = ('' + str).replace(/\D/g, '');
     
        //Check if the input is of correct
        let match = cleaned.match(/^(1|)?(\d{3})(\d{3})(\d{4})$/);
     
        if (match) {
            //Remove the matched extension code
            //Change this to format for any country code.
            let intlCode = (match[1] ? '+1 ' : '')
            return [intlCode, '(', match[2], ') ', match[3], '-', match[4]].join('')
        }
     
         return null;
    }

Simple add the function “formatFieldPhoneNumber” in the OnChange event of your phone attribute field in the Form passing the execution context as a first parameter and the field name (telephone1 for ex.) as a second parameter and try it.
 
Enjoy it!