Hello how are you, that icon indicates that your phone makes and receives high definition voice calls Voice HD, precursor of this new technology Currently some operators were already supporting voice calls of improved quality with a service called Voice HD.This service works with compatible 3G and 4G terminals in various situations, but we comment that they differ in concept of implemented in Android 5.1. Most operators already offer their HD Voice service and promises to work with compatible devices according to the codec used and increases the available bandwidth (bitrate) to transmit the voice as well as use a higher frequency of sound capture as you can see in the following graphic (traditional blue calls, HD voice calls, purple). https://cdn2.yoreparo.com/uploads/imagen_post/0003/82/4bd94a0242e2a22457086c6bf176d782132e705e.png" alt="" />