When you set a format, the number entered must match that format.
you can force the number to be entered to match the format you wish to use by turning the character mask on:
var Appointment_request_group_Phone_Spry = new Spry.Widget.ValidationTextField("Appointment_request_group_Phone_Spry", "phone_number", { format:'phone_custom' , pattern:"00000 000 00?", useCharacterMasking:true, validateOn:["blur"]});
really, the problem you are bringing up is a limitation to the Adobe spry phone validation. if you wish to be able to allow phone numbers from any country, you would be better of using numeric validation instead of phone validation.