Navigate to System Preferences > Security & Privacy > Privacy.
From the list on the left, select Microphone. You'll see Dialpad in a list on the right alongside other installed apps.
Choose the empty checkbox for the Dialpad app to allow it to access the built-in microphone.
macOS Mojave, released in 2018, strengthened user security and privacy by barring apps from using the built-in microphone or FaceTime camera without your explicit consent. It applies to all versions of macOS from Mojave and later.
Navigate to Settings > Privacy > Microphone.
Choose the toggle for Allow Desktop Apps to Access Your Microphone. It will allow the Dialpad app and other desktop apps to access the built-in microphone.
Please review this Microsoft Support article to learn more.