Carriers who assign numbers to SIMs in advance could set that field directly. Others, either because they don't assign a number at the time of the SIM manufacture/personalization or just because they can't be bothered as it's not functionally necessary will leave it blank - in that case from my experience iPhones will populate the field with the number they get back from the iMessage & FaceTime provisioning step but again that's not actually necessary for functionality. The field is also user-editable in Settings -> Phone if you wish.