You don't answer your phone, so '1571' does it for you. At that moment, the 'robot' recognises that the call has been answered and so starts the recorded PPI message.
However '1571' isn't 'listening' at that point because 'he' is saying: "Sorry, the person that you've called is unable to answer the phone. Please leave a message after the tone (etc)".
So, by the time that '1571' starts 'listening', the robot's message is already coming to an end.
I get exactly the same (part) messages too!