If it is a Skype or a third party voip provider(over Internet). The SIP call(including the REGISTER/INVITE and also the Voice packets) goes over the default PDN over LTE as it goes over any other network and just as any other traffic goes over LTE.
But in the case it is operator provided Voice services, where the Voice n/w infrastructure(IMS) is deployed by the operator himself, typically, there would be a additional PDN deployed, dedicated to Voice services. Additional PDN means additional PDN-GW(call it IMS PDN-GW)
The IMS PDN-GW shall connect to the IMS via the P-CSCF.
When the UE is powered on, the UE shall attach to both the PDNs.(the default PDN for best effort traffic AND the IMS PDN for Voice). The default bearer on the IMS PDN is established with QCI val 5(the QCI value required for SIP signaling). Note, that this EPS bearer will not support the QoS required for voice traffic, so we can expect to need an additional EPS bearer.
So, when the UE launches the Voice application, the SIP REGISTER procedure will occur basically Registering ( includes authentication and authorization) the user. The user may be identified by his MDN, MDN@domain.com etc, this identity is known as Public User Identity(IMS).
In order to do this, the ISIM will either be pre-configured with the P-CSCF IP or learns it via the DHCP process/assigned by the IMS-P-GW(in the Initial context setup request msg)
The LTE network doesnt recognize the SIP protocol, but as it is occuring over the IMS-PDN, it lands up on the IMS PDN-GW which forwards it to the P-CSCF.
When the user dials a call, the SIP INVITE/100/180/200/ACK (signaling related to setup of Voice call) shall occur. During this procedure, the UE and the P-CSCF perform QoS and resource negotiations using the SDP(Session description protocol), which is tunneled with the SIP messages. The P-CSCF learns the QoS requirements of the call and contacts the PCRF (over the Rx protocol) for the actionable charging and QoS rules.
PCRF then forwards these across the Gx interface to the Policy and Charging Enforcement (PCEF) that lives with the IMS P-GW in the LTE network. This is the first time that LTE is aware that it must support voice traffic.
The P-GW now takes charge and sends a Create Bearer Req to SGW(and to MME), MME sends to eNB, ERAB setup request for EPS dedicated bearer setup to establish a separate “dedicated bearer” (with a QCI value of 1) toward the subscriber that can handle the QoS requirements of a voice/video call.
The eNB sends RRC reconfig (with the new EPS bearer ID, DRB ID, Radio configs - RLC/PDCP/MAC/PHY, ESM pdu).
In the RRC reconfig, it also sends SPS(Semi-persistent scheduling) configuration info, to tell the UE to use the SPS-RNTI for the scheduling grants. SPS is a type of scheduling, which in a nutshell, enables the eNB to grant to the UE the resources for a fairly longer period of time considering Voice traffic needs constant bit rate and low jitter, therefore saving time involved in UE negotiating for resource more frequently as in Dynamic scheduling.
Also at this point of time, the UL-TFTs are set on the UE.
UE sends RRC reconfig response followed by ESM-Activate dedicated EPS bearer accept, that reaches the MME
MME now sends a Create Bearer Response to complete the establishment of the dedicated bearer between the UE and PGW.
At this point DL-TFT(traffic filter type) is established on the IMS PDN-GW, which is a set of rules to tell the PDN-GW to match the packets coming from IMS nw to the appropriate bearer.
At this point, the UE ESM knows about the Dedicated bearer being established and this is communicated to the SIP User agent in the UE, which sends a SIP UPDATE message to signal the QoS parameters taken effect to the IMS.
When the call is torn down, either from the IMS or the UE(using SIP BYE msg), the P-CSCF intimates the PCRF to release the resources. PCRF commands the IMS-P-GW to delete the dedicated EPS bearer.
A note however. The voice packets per se, do not traverse through the IMS network. IMS is only for the signaling. The User plane elements such as MRF and Media-GW entities handle the routing of RTP packets, codec allocation etc.