Check your intelligent wi-fi settings: settings/connections/wi-fi - tap the dot menu and select "intelligent wi-fi and turn off "switch to mobile data". However, if this setting is on, and WhatsApp keeps switching, it means that your wi-fi network is unstable or experiencing slowdowns. It will only switch to mobile when there is an issue with wi-fi or wi-fi is not available. If you turn this setting off, and your wi-fi is unstable or slow, you may not be able to use WhatsApp at all.
I suggest doing a speed test on your wi-fi to make sure it is working properly and running at speeds that would support voice and text. Also check with your ISP that you are getting a stable connection.
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