Good Way to Avoid Tele-Sales/Marketing

I'm sure a lot of people would agree with me that it is quite annoying to receive calls from various organizations (mostly banks) during the day to sell you all kind of things (ranging from insurance, so-called special promotions, loans, to lucky draw with free trial membership at fitness center) ... and somehow, they always know when to pick the right moment, such as when you're having a conference call, meeting, or just simply busily trying to get something out.

Recently, it has occurred to me that I can at least end the call very quickly by simply answering the phone with "hello" (in English).   Before, I would have switched to Cantonese channel after the other side doesn't respond with 2 hellos ... but turns out that if I don't switch channel, the tele-sales/marketing folks would simply just hang up.

Guess those commercials/promotions on TV about learning English ... are not that effective.