We don't receive many of these sorts of calls in my office. Those that we receive are mainly from building services, conference venues, directory publishers and solicitors.
I am always courteous to everyone on the telephone, although it can sometimes be hard to have dialogue with someone who is following a script! My boss takes the view that she won't talk to anyone who won't talk to me, and naturally doesn't want to be bothered with unnecessary calls. A Chief Executive is far too busy doing other things!
If the call is about something that is not of interest to us, then I say so, politely but firmly. If it is interesting to us, then I ask for details by email or post, but I never make promises to companies. At most, I can "promise" to pass on their information to a colleague in another department, if that is relevant, or to keep their details on file. Conference venues, in particular, are of interest to us, and we have retained the services of a couple of law firms to deal with specific projects, whose representatives just called us.
What tips me off that someone is trying to sell us something? That can be difficult because some of them are just plain devious

. I think one just develops the right frequency on our lovely admin-tracker-antennae.
I always try to think of the other person who is calling. I may not "owe" them anything - except for the normal courtesy that I would extend to anyone - but they have a job to do. If they are really obnoxious - then I can be a little bossy.
Raindance