First thing first: Is your account a prepaid or postpaid one?

If you keep the SIM card active (continue to make it officially yours, whether if it's prepaid or postpaid), the number will still be yours, and continuing to use it as your Whatsapp number in your phone will not be a problem at all.

If you give that number up (either through having it expire if it's a prepaid, or cancelling it before you go overseas if it's a postpaid), then the telco will recycle the number and someone else will become the new owner.

When the new owner uses this number to register for Whatsapp on his phone, he'll get your friends' messages if they continue to Whatapp you via this number.

Therefore, to answer your latest questions:

- The new owner will not know your contact list, but will get messages from your friends if they don't know that you've given up your old number and still continue to message to it.

- If you're using a smartphone and your contact list is still maintained in it, they'll still show up even if you use a new number to register for Whatsapp. In other words, when you open the contact list in Whatsapp that's registered with a new number, you'll still see your old contact list (just REFRESH the list, just to be sure).

- So long as you make sure that the old local SIM card continues to be in your name throughout your overseas stay (that is, you continue to reload for prepaid or pay your bills for postpaid), then there won't be another SIM card with the same number. It doesn't matter if you bring this SIM card with you or not.

Hope the above is somewhat clearer. 

My SIM card is digi prepaid.. Yup, thanks for the explanation.. It is clear enough