Summary
No Audio through the handset due to improperly plugged-in connectors.
Solution
Make sure that all multi-pin connectors are plugged in securely and all the way. If the bottom connector is not fully plugged in, for example, audio will work through the base but not through the handset.
If connectors were unseated and needed to be plugged in fully, please turn the power switch off and on again to reinitialize the unit.