The present document studies requirements, architecture, and possible functional enhancements of volume based charging for VoLTE. The study investigates how to associate the identifiers of caller and callee with volume based charging for VoLTE. The study investigates how to support the volume based charging for different IMS based services (e.g. CAT and voice calls, 3PTY conference, Call waiting, Call Hold). The study investigates how to collect accounting information to determine the status of VoLTE service delivery in PS offline charging. The complete or partial conclusions of this study form the basis for the normative work and/or for any further study.