Really? THAT'S your best advice? What about simply not answering the phone or investing in a call blocker? Both options are more likely to work and won't harm your own hearing.
You'll only be hurting your own ears. The modern telephone handset and phone systems do not have the capability to send or produce a sound DB level enough to hurt or damage someones hearing.
While it might be annoying to the caller, it doesn't harm them, and most likely it "clips" the speaker in their headset so they don't actually hear it.