Issue:
When a headset is attached to a Digium phone, the phone will default to using the speakerphone instead of the headset as the default answering and call placing method.
Solution:
If you would prefer to the headset to be the default device for placed or answered calls you will need to do the following:
In the Menu for the phones, navigate to Preferences --> Call Answering.
Set the option to make the headset the default answering device to Yes. Calls will then be played over the headset by default when answered or placed.