El día de ayer salió una actualización para el cliente de Windows Phone, siendo interesante resaltar las 3 opciones para poder hacer las llamadas

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.
This option is perfect for those of you who always want to have the richest conversation possible, including escalating voice calls to full video and content sharing. 

• VoIP over Wi-Fi only—Again, 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.
  This option is excellent for those of you who don’t want the Lync call to be placed using mobile data, which depending on your service plan could be expensive.

• 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.
  This option is the most reliable way to ensure that calls using Lync mobile don’t connect using your data plan and will be consistent when moving between networks. At the same time, it disables video and content for the call.

Para mayor detalle les dejo la liga del blog de Office

http://blogs.office.com/2014/12/10/lync-windows-phone-update-settings-voice-calls/

Espero les sea de ayuda.