Elf

whatsapp registers your "account" name as your mobile number. that way any of your friend has you in their contact list will automatically see you.
however. if you have installed whatsapp and registered it with phone number X on your iphone, took the sim card out and swapped phone number Y in, whatsapp will NOT automatically renew its registered account number to Y. you will have to go to setting > chat setting to change your number. it will send another SMS to the new number Y to validate you are actually owning this number.
In this case, your friends who have your number X in their contact book will cease to see you on their contact, while people who have your number Y will now see you.