On this phone, the power button is also the End call button. A short press ends the call. This is by design.

Press the Back button. That’s the physical key with the back arrow, at the top right of the keypad.

To switch to another app during a call, press Back, or hold Back for 2 seconds to open your recent apps.

If your screen stays on during calls, open the Kosher Qin Phone app → ☰ (top left) → Tools → Turn off Screen While on a Call, if your Tools page has it.

You’re probably using the Google Phone app. Switch back to the pre-installed phone app: Settings → Apps & notifications → Default apps → Phone app. If it still happens, clear the phone app’s cache: Settings → Apps & notifications → See all apps → [phone app] → Storage & cache → Clear cache.