Tried to call back comes up invalid number ?

Claiming I was involved in a car accident.

Tell them you don't have a car and they hang up.

This mob have been stalking me for 3 weeks now. They recon I have had a car accident and I'm entitled to make a claim for compensation. I played along with it the first time, but now it's really annoying me. I have told them 4 times now, not to call me again, but they keep persisiting. I have "True Caller" installed on my phone, but it doesn't recognize it as spam. When I call their number back, the phone is disconnected. They obviously use different numbers all the time, sometimes their number doesn't display at all.