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Tried duplicating the issue. You are right, it doesn't ring but also it is not going to voicemail either. It starts ringing if the normal call is still active and I disconnect the WhatsApp call, or vice versa.
To my surprise, I just tried entire thing on my Android device. Started a whatsapp...
Most probably you can... I just checked on iOS 12 and it does have an option to add a Yahoo account in the Settings App. Give it a try, I don't have an iPhone 4 to check myself.