I get a phone call from India trying to sell Viagra or Cialis

I have been receiving these calls for 3 years.  I've only answered 2 or 3 by mistake.

How are getting these people to keep calling for 3 years without a single purchase?

BTW - There are phone services called ? Tunneling:
 A person in India calls a local number in India ->
 The call is sent to a phone machine in the US ->
 The phone machine dials out on a valid US number

I'm pretty sure that is how they are doing it.  If the FCC gets the person offering the tunneling in America, I'm sure India just moves to a different tunneling provider.

I've received over 15 calls in the past few months from "Canadian Pharmacy".  The last two times were from 718-236-9842.  Other numbers include 203-411-2113, 203-411-2115, and 203-411-2110

Been receiving several odd calls lately from different 203- area code numbers, as well as this. No messages left, but once someone said they were calling in regards to college information which I had supposedly requested online.

We have had a ridiculous number of calls from this number in the past couple of months, when they called the last time they said a number as if some kind of secret code. We are in Glasgow, Scotland so not just happening in U.S.