Sorry if I didn't make it sound serious. Data theft is pretty alarming -- and we don't take it lightly. We definitely don't sell your contact information to other firms. We do work with third party organizations like NSDL, CDSL, KRA organizations (to check your identity), to process your account.
Whether a leak is happening there or within the org, it has got to be found. If you have any evidence of how they came about your number, do let us know.