You can tap the cover screen and swipe to the right to answer a call or swipe to the left to reject it. If you aren't using a headset or Bluetooth audio accessory, the speakerphone will be enabled when you answer a phone call with the phone folded. You can also use Bixby to help you make calls. Simply say, "Hi Bixby, call Mom."
How to setup Bixby to use when your phone is locked
Step 1. Unfold the phone and navigate to Bixby Assistant Home.
Step 2. Open the app, tap More options (three vertical dots).
Step 3. Tap Settings, and then tap the switch next to 'Use while phone locked.'