The initial stages of setting up a VoLTE call are the processes of the initial attach, P-CSCF discovery and creating the default bearer for SIP signaling (by registering with the IMS network and subscribing to a registration event
package).
The first step in a VoLTE call setup is a SIP INVITE request initiated by the calling UE. Following this step, agreement is made on the media-specific parameters such as codecs (e.g. AMR or WB-AMR). After some RINGING, TRYING and OK messaging, the calling UE may respond with a Provisional ACK (PRACK) method as shown in the flow diagram above and as defined in RFC 3551. The PRACK method is used because ACK cannot safely traverse proxy servers that comply with RFC 3261. The PRACK is also forwarded to the called UE. When the called subscriber answers the call, the called UE will respond with a 200 OK before the RTP (media) messaging begins.

In a VoLTE call, the bearer is associated with a QoS Class Identifier (QCI) of 1. QCI values from the 3GPP’s TS 23.2032 are shown in Table 1. Each is generally targeted to a specific service type based on delay and packet
loss requirements. For example, a video telephony call might add a second dedicated bearer for video traffic, assigning a QCI of 6 to that bearer.

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Source: IMS Procedures and Protocols: The LTE User Equipment Perspective by Spirent