When "Do Not Disturb" mode is activated, all calls are muted on the LG unit. Call exceptions can be set under the priority. You can choose to allow all calls, from contacts or only from favourites. Calls from the selected group can be heard even if "Do Not Disturb" mode is activated on the LG unit.

Note: The Only with priority mode must be activated.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound
Step 3:  Tap on  Do not disturb
Step 4:  Tap on  Select priorities
Step 5:  Tap on  Calls from
Step 6:  Choose between  No one ,  Everyone ,  Contacts  and  Favorites
  1. Open the Settings
  2. Tap on Sound
  3. Tap on Do not disturb
  4. Tap on Select priorities
  5. Tap on Calls from
  6. Choose between No oneEveryoneContacts and Favorites

LG Instructions