Forgot to mention, the 'leave phone on side' approach being chosen for its 'tar pit' like qualities - not only did it improve the quality of my life by ending the call immediately (from my point of view), but it also improved the quality of life of anyone the telesales person didn't manage to call that day due to wasting their time waiting for my mum to pick up :)

Ah, yes; along the same lines as telling the canvasser for your party to go away immediately but inviting their opponents in for a cup of tea and a nice long argument. Except that in this case it doesn't cost you any of your own time either :-)