Welcome to Apple Support Communities. I understand that you're having trouble with using the number keypad in the Phone app. I see that when you try to use this your screen either goes black or is unresponsive to your touch. I know how important it is to use your iPhone to make calls, I'd like to help. First, if you haven't already, I'd recommend restarting your iPhone: Restart your iPhone, iPad, or iPod touch
If the issue continues after restarting, create a back up of your data using these steps: How to back up your iPhone, iPad, and iPod touch
Then follow the steps below to reinstall the iOS software to your iPhone. This process will erase the software currently installed and replace it with a fresh copy. Please be sure to closely follow step four and choose to "Update" in order to prevent data loss:
If you can‘t update or restore your iPhone, iPad, or iPod touch
- Make sure that you're using the latest version of iTunes.
- If iTunes is already open, close it. Connect your device to your computer and open iTunes. If you don't have a computer, borrow one from a friend or go to an Apple Retail Store or Apple Authorized Service Provider for help.
- While your device is connected, force restart it with these steps, but don't release the buttons when you see the Apple logo, wait until the connect to iTunes screen appears:
- On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the connect to iTunes screen.
- On an iPhone 7 or iPhone 7 Plus: Press and hold the Side and Volume Down buttons at the same time. Keep holding them until you see connect to iTunes screen.
- On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons at the same time. Keep holding them until you see the connect to iTunes screen.
- When you see the option to Restore or Update, choose Update. iTunes will try to reinstall iOS without erasing your data. Wait while iTunes downloads the software for your device. If the download takes more than 15 minutes and your device exits the connect to iTunes screen, let the download finish, then repeat step 3.
- After the Update or Restore completes, set up your device.
After you've completed these steps, test to see if your iPhone still displays this behavior when you're trying to use the keypad to make a call.
Best Regards.