Just don't answer the phone.. Nothing to worry about.. If you don't know the number just don't answer the phone. They won't be in business if no one answers the phone.
I recently realized my phone includes a call block feature. When I add a number to the call block list, the phone would not ring and instead issue a fake disconnected signal. This is supposed to tell the calling system the number is no longer in service and they would hopfully flag it as such.
This is a landline Panasonic.
Btw, I got a call today from robo Rachel and card services.