Why does WhatsApp show "silenced voice call Focus mode" during ongoing call?

i have an issue with a friends WhatsApp call. What happened was, when my friend received a call, he remembered he was talking to that friend for 5 minutes but from the WhatsApp message, it shows "Silenced voice call Focus mode".

How anyone know how did that happen? i am trying to recreate that scenario but unable to do so.

[Re-Titled by Moderator]

Original Title: whatsapp slience voice call focus mode