Google continues to integrate their Google Plus platform with their wide range of services.
Google Voice is the latest to get Google Plus integration. Google Voice now supports the Google Circles.
Google spoke about this feature addition:
Circles give you more control over how you manage your callers; for example, calls from your “Creepers” circle can be sent straight to Voicemail, only your “College Buddies” circle will hear you rap your voicemail greeting, or you can set your “Family” circle to only ring your mobile phone.
You can customize your Circles settings by visiting the Groups & Circles tab in your Google Voice settings.