In a signalling network employing the SS7 signalling protocol, it is known to use masks to mine CDR stores in order to obtain data for analysis in respect of a particular customer of a communications network associated with the signalling network. However, the use of masks to mine the CDRs can result in the retrieval of unwanted CDRs or the omission of relevant CDRs. The present invention therefore provides a call monitoring apparatus that receives a CDR and accesses a reference database, comprising a list of customers and respective telephone numbers, in order to identify an association between the CDR and a given customer. Consequently, speed and efficiency of analysis of CDRs is increased.