Call centres now have to leave a message in the case of a silent/dropped call (in theory to stop scaring older people who get lots of calls where there are hung up on). Best thing I can think of there is if they tell you the name of the company who is contacting you then look up their website and send an email telling them you don't want to receive any more unsolicited calls. Once they have received that then they have to add you to the DNC list.
