Can't receive verification text to move what's app to new phone

Hello, I'm trying to move my what's app (inc all history) from my iPhone 6s to my new iPhone SE (8). I put the sim card in the new phone and downloaded various apps, others have moved without problem. I've downloaded what'a app and it's asking for the 6 digit verification code which it says it's sent to me. I am not receiving the message on either device and therefore cannot move what's app. I am competely stuck. I ask it to call me with the code instead and nothing happens. Please could anyone assist me? Many thanks.