How is this possible!

Me and my wife took out a new contract in July. We got new sim cards but mainly use our old sim cards. But when I call my wife on my new number it goes through as me, but as soon as she tries to call me on the exact same number another unknown person answers. The WhatsApp aswell when she goes on to WhatsApp to my new number somebody else is on the profile picture and the account is still active. I do not get any sms on the new number aswell and I can't check my balance. How is this possible please sort this out.