there are different network technologies how cellular phones communicate with the network towers, in the US Verizon & Sprint used CDMA, At&t & T-Mobile used GSM.
Now that the new technology LTE evolved everyone caught on to LTE, as of now only for the data, talk & Text still run on CDMA or GSM, one of the drawbacks are that you can’t use Talk ant Data at once.
VoLTE “voice over LTE” or as verizon calls it advanced calling, utilizes the LTE for talk their for you can Talk & browse at the same time just as you would open 2 tabs in the browser it. (verizon counts it as talk even though its transfered through LTE)
Google Fi singed a contract with T-Mobile & Sprint for the LTE only as mentioned earlier the are using VoLTE.
in a few years from now this would be the only way to call till some better transfer protocol is launched.