Hello akbarfromanantapur,
You mention the "call alert slider" is unresponsive when you want to answer a call. This isn't expected behavior and there are some steps you can take that should help.
Restart your iPhone
If the issue persists, go to Settings > General > Software Update and install any iOS updates: Update your iPhone or iPad
Update your iPhone or iPad wirelessly
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now.
If the slider to answer a call still isn't responding, you'll next want to check with the Apple Support team. You can reach them here: Get Support - Apple
You can also find the support phone number for your region at this link: Contact Apple for support and service - Apple Support
Take care.