When VoLTE is available, subscribers could initiate a call using the LTE system but drive out of the LTE coverage area. Systems must be able to hand that call off to a traditional voice network. The mechanism for this, network software called circuit-switched fallback (CSFB), is now available on most networks. Another issue is getting VoLTE into the handsets. VoLTE requires a separate chip in the phone, and few phones have such a capability today.
LTE uses the Adaptive Multi-Rate (AMR) vocoder, which also is used in GSM systems and other 3GPP standards. It has a variable bit-rate capability from 1.8 to 12.2 kbits/s. Digitized voice is then assembled into AMR packets and then into IP packets that are scheduled into a transmission sequence. A call is allocated to some of the OFDMA subcarriers and to some of the time slots within the bit streams of each subcarrier.
I'll share some thesis about LTE.