I'm guessing you've signed up to something online using your mobile number. I always use my landline for that as I rarely answer it, unless I'm bored and fancy wasting a bit of their time.
Confuse them.
Caller: Hello, I'm calling you about your recent accident.
Me: Which One?
Caller: Er.. Your recent one.
Me: Which one? I've had a few, I'm very accident prone.
Caller: Umm, your recent one.
Me: Well, you're the one contacting me, you'll have to tell me which one.
Caller disconnects.