A method for calculating a mean opinion score (MOS) during an ongoing Voice over Internet Protocol (VoIP) call is provided. The method may include determining a time delay between a VoIP source and a VoIP destination connected by a communications network. A start recording message is sent from the VoIP source to the VoIP destination. A first recorded call sample from the VoIP source and a second recorded call sample from the VoIP destination are generated, whereby the first and the second recorded call sample are generated with a recording delay value corresponding the determined time delay for synchronizing the first and the second recorded call sample. Using an intrusive call quality measurement, a first MOS value is calculated based on the first and the second recorded call sample. Using a non-intrusive call quality measurement, a second MOS value is calculated based on the first MOS value.