Lumia 640 dual sim, and when people call me it says ''unknown caller'', why?

[WPCentral Question]

Lumia 640 dual sim, and when people call me it says ''unknown caller''

I have a Lumia 640 dual sim, and when people call me from a landline or a mobile, it says ''unknown caller''. When people call me with Whatsapp, then I recognize the caller. How to fix this error?
Currently, there is only a sim in one slot, not in the other one. All contacts are saved on the memory card.