I need help my iPhone 4 it's not adding new number on WhatsApp

Hello! I am in Rwanda, I have iPhone 4 , I have tried to download some applications it's not working, I tried to sign in iTunes & apple store now the system is asking me iTunes gift card code I don't know how to find it. And it's not possible to add new number on WhatsApp the system is asking to use the last number used on this phone , this iPhone 4 is second hand it was used by many people I can't find the number used on WhatsApp of this iPhone what can I do? Serial number of the iPhone 4: 79****A4S

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