I was having the same issue on an Samsung A7 2018. During a call, I could hear only text messaging notification and nothing else (not WhatsApp nor e-mail notifications. I figured out that the the problem was the Samsung native Phone App which came installed on the phone. My solution was to simply download Another Phone app (Drupe) and now I hear all text messages, WhatsApp messages and e-mail messages during a call. I hope it helped.
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