Daily Harrassment

This company harasses me every day because I let an old pensioner use my phone to call them. I have informed them of this yet they keep calling me from 7:00 am to 8:00 pm more than 3 times a day. .. this is not my debt. How do I make them stop? Do I have to change my number ? They are so rude..they have threatened me by saying they will never stop . They put the phone down in my ear .. and when I do get to explain the situation they promise to send my number to the right department for deletion. This never happens.