Nashua Mobile
How did you get my number?

I received an unsolicited phone call from one Nashua Mobiles "marketing" people offering me a cell phone deal over the phone.
When I asked the operator where they got my number they informed me that they got it from the Direct Marketing Association of South Africa.
Now, I am fully aware that these companies often get their data bases from "unsavoury" sources and sell them to companies, such as Nashua Mobile for a small fee.
I am also aware the Vodacom (with whom I am a contract client) would not give my details to a 3rd party for any reason according to their privacy clause in their contracts.

So my question is, how did Nashua Mobile (a competition company to Vodacom) receive my number and think it in my best interest to call me on a private number?
Part of the reason for having a cell phone is that you have the option of distributing your private number at your own discretion, and I do not recall on ANY occasion giving my number to Nashua Mobile or ANY of these other companies that call me on a regular basis offering me insurance, furniture, pre-approved loans and the like.

Please explain!