Do buy a SIM card when you are in the United Arab Emirates (if you see a good deal). The main network operator is Etisalat, but there is also du (دو). However, if you are planning to stay less than a few days, then an e-SIM might be a better option (ask the network operator about it in your country).
If you use the free du SIM card they give you in Dubai immigration, then that might not work when you return to Dubai, (if your first arrival to Dubai was transit). If you are in Dubai for a day or two, then the du SIM card might be worth it.
When you leave Dubai via the e-gates, your free du SIM card will automatically stop working. That SIM card was registered to you when you arrived (despite it still being within its packaging).
When returning to Dubai, if you want to use the same number (that came with your free Du SIM card), then you will need to visit one of the Du company branches and it could take a while. You probably wont be able to use the same ‘free’ SIM card and may end up with a new number. There you can also buy credit (top-up), but it is only value for money for longer stays.
While in UAE, you wont be able to use WhatsApp to make calls (only texting will work). However, there are local apps that you can use for calling, e.g. Tawasal, Botim. Alternatively, a VPN would allow you call via WhatsApp.
Arabian Misadventures 