Until recently my desk phone at my workplace had only internal extension and could not be reached directly from outside. The call to my desk phone had to be routed through the front desk office or through the EPABX system. The moment DID (Direct Inward Dialling) got activated, the telemarketing calls has started coming in.
HSBC was the first one to sniff. Most of my officemates had got a call from HSBC for their free lifetime gold credit card offering.
How did HSBC get these numbers? Might be the service provider could have leaked. Otherwise nobody could sniff the number right on activation.