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Fault Localization (FL) is a critical task for operators in the context of e-TOM (enhanced Telecom Operations Map) assurance process, in order to reduce network maintenance costs and improve availability, reliability, and performance of network services. This paper investigates, in a practical perspective, the use of a well known FL technique, named codebook technique, for the IMS control layer of a real Next Generation Network, deploying wireline VoIP and advanced communication services. Moreover, we propose some heuristics to generate optimal codebooks, i.e. to find the minimum set of symptoms (alarms) to be monitored in order to obtain the desired level of robustness to spurious or missing alarms and modelling errors in the root cause detection, and we evaluate their performance through extensive simulations. Finally, we provide a list of some practical Key Performance Indicators, the value of which is compared against specific thresholds. When a threshold is exceeded, an alarm is generated and used by the FL processing.