Iphone purchased 2 year contract - can't get proof

My niece purchased an Iphone on contract and when time came to get a new phone she swapped to Samsung and gave me her Iphone. Iphone requested a password and I put MY password for MY simcard in - (she never knew I would need her Iphone password). Long story short, Iphone is locked and Apple refuse to reset the Iphone to default, so that I can use it with my simcard, - WITHOUT PROOF OF PURCHASE. They need the last 3 digits that appear on Vodacom statements which are blanked out. My niece phoned Vodacom and they informed that they do NOT keep record of the purchase. At first. After a lot of badgering she found someone who told her that she would get a statement with those digits on to her. This was in January. We are still waiting. We have been busy and could not pursue it. I need the phone. What can we do to resolve this?