Thanks for your response. I am aware that whatsapp is a third party app and that's kind of my point: it worked, when the phone app did not. I have no games on my phone - in fact I have very little. Also, the phone is responsible for memory management and I shouldn't need to "clean" apps for the phone to receive calls - that's a fundamental job of a phone! I have never had issues this bad with any other Android phone, ever.
I had some very similar issues which were due to network issues. In my case I had to contact customer support for my network provider and they did a "network reset". Since then I have had no issues.