whatsapp transferring to another phone?

I need to send my phone back for repair

I've got an old galaxy S2 that I was going to use while my other phone is getting repaired

I changed the sim over thinking that whatsapp is linked to the phone number, however it still seem to be linked to the phone that needs repairing as I was getting messages when it was connected to wifi

Any ideas how to change it to my old phone temporarily , then back to the repaired one?