First, UE establishes connection with IMS PDN using the PDC connectivity procedure. As part of PDN connectivity procedure, UE gets a new IP address. Once PDN connectivity procedure gets completed, UE starts with SIP signalling (all the SIP signalling messages exchange over new IMS bearer). As part of SIP signalling, application server based informs to PCRF regarding QoS requirement of new application and as resultant, PCRF initiates dedicated bearer creation with QCI "1". This QCI bearer 1 carries voip packets.
In short, PCRF is the node in LTE network which initiates dedicated bearer creation for the voice call.