Allow Blank Phone Number for courier integrations

Hi

Due to varying courier integrations we have to have both phone and mobile number fields populated to allow the shipping label to created.

However we have plenty of instances where we only have one or the other. Currently if a phone number is blank we add via an order rule 000000000 to the field to ensure we can create the label.

However due to the way the API external partner courier integration works it defaults to look at one of the fields to see if it's populated and if it's blank look at the other phone field. Well due to the requirement of being forced to add in 000000000 the fields are never blank and the API just sees a number field with a load of 00000000s