I get cold calls all the time and I just tell them that I'm registered with TPS and asked them if they would like me to pass their number on to TPS. As a rule they are very apologetic and tell me they will remove my number instantly. I had one that wasn't having no for an answer though. I think someone else told them to shut up and get off the phone in the end.
Of course it's not good and ethically it shows that they don't care about your life, but... we're not all the same and you will find that in all business. It's a chance you take and thankfully most of the time it works out well.
If you have asked her to stop and she doesn't then yes I can understand how that would be annoying. A bit like I've a client I never met who keeps texting me and has been doing so for 2 years now. I shall be throwing the sim card away when I move on though, so it doesn't really matter, just a minor irritant.