As published in the Lync blog today, Lync for Windows Phone has more settings for voice calls:
VoIP always—The device will use a Wi-Fi connection, if available, to place the call. If Wi-Fi isn’t available, the device will defer to the 3G/4G data plan and the Lync voice calls will be initiated over that connection.
VoIP over Wi-Fi only— The device will use a Wi-Fi connection, if available, to place the call. If Wi-Fi can’t be found, this setting instructs the Lync mobile service to connect you using Call via Work—placing a call to your phone first, then to the destination number.
Cellular—The Lync mobile service will directly place a call to your phone number using Call via Work and then complete the call to the destination.
Read the full article in the official Lync blog: