Old sim deactivated but i want to use that same number for new sim My old sim has been deactivated and most of my contacts know me by my old number.
I want some way to register my whatsapp to that old number
Phone numbers aren't tied so much to the SIM card, but to the provider via your SIM card. Your old phone number can be reassigned to another person now, so it would be unwise at best, probably illegal, to make Whatsapp use your old phone number.

In short, "your" old phone number is no longer yours.