Thank you for reaching out. I can certainly understand how frustrating it can be when our phones turn on in our pockets. When your phone is stored in a pocket or bag, your normal day-to-day activities can cause your phone to accidentally wake up or pocket dial someone. The "Lift to wake" and "Double tap to wake" features on your phone could be causing this. You can disable each feature individually, but you may experience better results if both features are turned off. Give this link a try: https://www.samsung.com/us/support/troubleshooting/TSG01200663/
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you.
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Oh no! I can definitely understand how concerning this is to have your mother's S21 Plus calling people without any prompts. If your phone or tablet is misbehaving, it's possible that a third-party app is causing the issue. To figure out what app is acting funny, you can use the Safe mode feature. In Safe mode, your device won't run any third-party apps. This allows you to easily identify and remove the app that may be causing the problem. Give this guide a try to see if a third-party application may be causing this very strange behavior: https://www.samsung.com/us/support/answer/ANS00062983/
Let me know if this helps!