Disclosed is a method for determining an authenticity of at least a portion of an audio stream of a voice call connection. In the method it is received a request for confirming an authenticity of at least portion of an audio stream of a voice call connection; detected a number of voice call connection specific watermark patterns included in the portion of the audio stream in question; and set, in accordance with a detection of the number of the voice call connection specific watermark patterns included in the portion of the audio stream, a detection result to express one of the following: i) the portion of the audio stream is authentic, ii) the portion of the audio stream is nonauthentic. Also disclosed is a network node, a computer program product and a system.