Usage of apps such as WhatsApp, Messenger and suchlike is pure data use - your phone handles it as data, as does the mobile network. "Calls" are in-app audio between other app users and are not routed through EE's network as calls in the same way as direct-dialled calls are.
With that in mind, what exactly happens when someone tries calling you? When was the last time someone was able to call you? Can you make outgoing calls?
As a first point, check you haven't inadvertently set a "do not disturb" mode or an unconditional "divert all calls" - both of these could explain your symptoms.
