It's unfortunate, but, there is no way to stop these calls. I run a coin store and they call our store, from different masked numbers, about once a week. They know we are a business, but, they don't care. The only real defense is to get call reject/block. Any conversation with them is a complete waste of time.
The Canadian pharmacy woke me up this morning ( I have an east coast phone number but live on the west coast). Now I know this is so immature but I called back using my hands free while getting ready for work, and my drive into work. I acted like a confused individual on my repeated calls. My last call lasted over 3 minutes and the sales perso sounded angry and threatened to put me on the "do not call list". I can only hope.
This call being a "Canadian Pharmacy" is an Urban myth. Think Caribbean, or Asia. Remember with a spoofed (faked) phone number your caller ID could say anything you want it to. Pharmacies in Canada are regulated by the Government. One operating illegally would be gone quick. Want to see how easy it is to set up an on line pharmacy? Do a web search on that topic. You can set one up in about 10 minutes. Now all you need is 3rd world horse pills to sell.
I took it a step further and acted as if I was a legitimate customer. Third grader stuff I know, but with a hands free and a hour long drive into work made the trip entertaining. Longest call was over 3 minutes and the telemarketer was pissed and threatened to put me on the do not call list.
Calling and engage them while on hands free while your doing something else. Waste their time and have fun with it.