Amendment 4 to Recommendation ITU-T Q.761 was produced to meet the need for the implementation of the customized alerting tone (CAT) service as specified in ETSI TS 123.205 (2009-07). It contains the modifications to Recommendation ITU‑T Q.761 (1999) in order to accommodate these needs. This amendment should be read in conjunction with the related amendments to Recommendations ITU-T Q.762 and Q.763.