Pn recent years, increasingly sophisticated ways have become available for accurately estimating customer opinions of PSTN call quality using non-intrusive objective measurements. This paper discusses the application of non-intrusive monitoring of end-to-end call quality to voice-over-IP (VoIP) networks. The problems and opportunities of a number of network scenarios are studied, including the impart of interconnections with the public switched telephone network. In particular, interactions between VoIP network elements, call signalling information, the voice signals and the choice of monitoring location are examined.