find the line that initializes the phone spry validation.
It will look likle:
var Registration_group_Phone_Spry = new Spry.Widget.ValidationTextField("Registration_group_Phone_Spry", "phone_number", { format:'phone_us' , validateOn:["blur"]});
and change it to use your custom pattern:
var Registration_group_Phone_Spry = new Spry.Widget.ValidationTextField("Registration_group_Phone_Spry", "phone_number", { format:'phone_custom' ,pattern:"00000 000 000", validateOn:["blur"]});
In the patern, you can change the last character to a '?' to allow any character, the will need to enter a space for at the end of the 10 digit phone number. for example:
pattern:"00000 000 00?"
would allow the phone number to be enter as "12345 123 12 " or "12345 123 123"