The third tab within the Dialer window is the Settings Tab.
There are two important Audio Settings:
Ringing Device - the system to ring when the call happens.
Output Device - the microphone and audio that you'd like to use for the call.
These are separate settings for a useful reason: You may want the ringing set in a way that you can never miss it, but you want to use a different audio/microphone for the actual call. For example, you may want smrtPhone to ring on your computer speakers so you always hear it, but you might want to take the call on a headset.