the spry validation code for the phone number field is set to restrict the maximum characters for the phone number field to 14:
var Phone_Number_Spry = new Spry.Widget.ValidationTextField("Phone_Number_Spry", "phone_number", { format:'phone_us', maxChars:14 , validateOn:["blur"]});
the format:
x (xxx) xxx-xxxx
is 16 character long though, to use that format, you will need to change the spry validation to allow for 16 characters.