"Can't do anything else" meaning there's no data service?
If the above is true, make sure your number has "Advanced Calling 1.0 / Advanced Calling 2.0" enabled. Then, go to Settings > Advanced Calling and make sure HD Voice is enabled. With this setting, so long as you are in an LTE service area and your phone is connected to LTE, you should be able to use Voice and Data at the same time.
No matter what, the phone will still allow you to use apps while on a call, if you're on Wi-Fi. Apps which don't need data connectivity should continue to work even with a call going.
The exception to this rule is if your phone has a lock code enabled. To get to the apps, you'll need to hit the home button to get to the lock screen, and then unlock your phone. This shouldn't disconnect your call. Also, if your phone is in "Ultra battery saver" mode or happens to be set up to "Simple" mode, multitasking features will often be disabled. Both of these can be turned off from your Settings, or Notifications bar if they are enabled.