This Recommendation provides transmission performance guidelines for voiceband services over networks that are an interconnection of an IP network (e. g. the Internet) and the PSTN. As modern telecommunications systems continue to evolve, hybrid networks of various kinds will become more prevalent. This tendency, together with the reality that voiceband communications will be an important source of traffic on such networks, has led ITU-T to develop this Recommendation. The intent is to provide to a wide audience a set of principles on transmission performance aspects of hybrid Internet/PSTN connections.