iphone 4s says called failed everytime i call a particular number

So I recently upgraded my iPhone 4S to a 5S.

I gave the 4S to my wife.

Now the weird thing is that whilst I am able to call and text her from my 5S, she is not able to call me or text me. If she tries to call me, it just says Call Failed. It's almost like the phone has reminents of the old number and is not allowing her to call the number which used to be on the phone.

Both phones work fine calling and texting other numbers.

I have looked online and done all sorts of things.. transferred to new SIM cards, reset the phones, backed up and restored the phones, reset network settings, check that there was no barring etc.

I cannot figure it out and it's so annoying that it's not working. The only thing she can do is to iMessage or Facetime me.

Does anyone know what has happened? Is the old phone clinging onto the old number someone and not allowing connectivity to that number? Can anyone help?