remove this:
pattern="\(\d{3}\) \d{3}[\-]\d{4}"
and remove this:
onBlur="WAValidatePN(document.getElementById('CheckoutWizard_NewFromBlank_Default').Phone_Number,'',false,true,'x (xxx) xxx-xxxx',document.getElementById('CheckoutWizard_NewFromBlank_Default').Phone_Number,0,true);"