Well, I don.t know if my experience will give you any insight, but here goes.
I had an old Samsung Galaxy S4 that I wanted to use in the UK for several days. I bought a prepaid UK sim, and installed it. Then I installed What's App. Since my phone had all my contacts on it, I figured that it would pick up the ones that had What's App. Nooooo... Only two of contacts appeared, but I knew that several more had What's App. I went crazy trying to get my other contacts to appear. I thought that I must have once installed What's App, and that it was vestiges of the old account were lurking. As it turns out...(leaving out hours of trouble shooting here)
I finally deleted the account under the UK number, uninstalled What's App on the phone, and reinstalled it using a U.S. number I have on a dumb phone. Worked fine, all my contacts EXCEPT THE TWO THAT ORIGINALLY CAME UP were there. Could not figure out why, but they were two people I hadn't spoken to in years, so decided not to worry about it. Then I tried changing the number to the UK Sim. It gave me the option to only notify certain people on my contact list of my change in number, so I only clicked on a few. When, What's App FINALLY allowed me to reactivate the UK number, the contacts that I had clicked came up, AND THE TWO THAT DIDN'T COME UP WITH THE U.S. NUMBER. Upon a closer look, the two contacts that kept appearing weren't my contacts...they were complete strangers with the same name that have UK numbers, even though I'm quite sure both of the my contacts are in the States, and those UK numbers are not in my phone. So strange.
So yesterday I had a friend download What's APP and I put my sim number into her contacts. I popped right up in her contacts. But, on my phone with the Sim, nothing, even though I put her number in my phone in international format and refreshed my contact list. So, I thought that's What's App accessed the contact list on my phone, and determined which numbers on the list are registered with What's App, and then those contacts would pop up as a what's app contact. But something else is going on. For some reason, What's App can't access my contacts with the UK Sim in it.