One of the ways your bank tries to protect you from loss is by calling you if it sees transactions that look suspicious. All well and good except for a couple of things:

1) Thieves who can take control of your bank account can tell the bank you have a new phone number, and then answer the calls on your behalf.

2) Banks who are smart enough to call your previous phone number if your number was recently changed can be blocked from reaching you.

A new variation, primarily applied (thus far) to high-value thefts, uses automated calling programs to call your previous numbers, delivering dead air, advertising, and other such things. The calls continue incessantly (as much as every 30 seconds for a month). This prevents the bank from checking with your old number, and allows the thief more time to convince the bank that they do have the rights to take money out of your accounts.

See the original article in Wired magazine here.