Details about the deal:
16.5. All electrical free gift claims are initiated by sending us your first two bills within 28 days of the date of the second bill. Bills received after this date will not qualify for their gift. Bills must show no outstanding money from previous bill and also show no changes to your tariff (e.g. T Mobile Flext 35 changed to T Mobile Flext 30). Please take copies before you send these bills to us as we will not be able to return them. Please allow up to 28 days for delivery of your gift.
Seems pretty straight forward