For Unknown reason not known to the public yet. It seems Zicta has blocked the IP Addresses used by Whatsapp, hence making Zambian users not to access Whatsapp. As we gather information to find out why Whatsapp has been blocked, Use a VPN to use it.
When you download and install the VPN app on Playstore, Visit the VPN app and click connect, Once You are connected, you can send/receive messages on Whatsapp.