While the LTE circuit switched fall back (CSFB) can be considered as a procedure separated from voice service delivery over LTE, it has an impact on the QoS for the voice services and is therefore worth of attention and consideration. Indeed, operators have to make sure that clients purchasing LTE-capable devices must keep the level of QoS they used to have with their former 2G and 3G devices, even when VoLTE is not available (for any reason: network, device). Therefore, specific and complementary KPIs are proposed in this Recommendation, together with procedures for their assessment. It should be noted that this Recommendation focuses only on the specific impact of CSFB, the information it contains must be considered as a complement to provision already provided in the Recommendation ITU-T G.1028: "End-to-end quality of service for voice over 4G mobile networks".