WhatsApp, in its FAQ section, has revealed the list of features that will be limited. Users who do not sign up will not be able to access their chat list. They can, however, answer incoming phone and video calls.
Users who have their WhatsApp notifications enabled can tap on them to read or respond to a message or call back a missed phone or video call. After a few weeks of limited functionality, users will not be able to receive incoming calls or notifications. WhatsApp will also stop sending messages and calls to your phone.