Placing one's number on the National Do Not Call Registry will stop most, but not all, telemarketing calls.

• A person may still receive calls from political organizations, charities, telephone surveyors or companies with which he or she has an existing business relationship.
  
• The registry only applies to residential lines, not to business lines.

Only telemarketing calls are covered — that is, calls that solicit sales of goods or services. Callers purporting to take a survey, but also offering to sell goods or services, must comply with the National Do Not Call Registry. An example of an operation attempting to skirt this rule is the Dove Foundation which places "survey" calls and then requests permission for a follow-up call, which attempts to sell goods through a for-profit company. This operation resulted in a restraining order in Missouri in March 2006, but the operation continues in other states.

Even if one puts his number on the National Do Not Call Registry, a company with which the person has an established business relationship may call for up to 18 months after his/her last purchase or delivery from it, or last payment to it, unless the party specifically asks the company not to call again. (In that case, the company must honor the request not to call. If they subsequently call, they may be subject to a fine of up to $11,000 USD.) Also, if one makes an inquiry to a company or submits an application to it, for three months afterwards, the company can call. If one makes a specific request to that company not to call them, however, then the company may not call, even if you have an established business relationship with that company.

If one does not want to put a number on the national registry, they can still prohibit individual telemarketers, one by one, from calling by asking the telemarketer to put them on their company’s do-not-call list.

Despite a spate of emails warning to the contrary, cell phone numbers need not be included on the National Do Not Call Registry to avoid unsolicited calls. FCC regulations already in place prohibit telemarketers from calling a cellular phone number with an automatic dialer. Experts tell media sources that the false email turned urban legend probably started because cellular companies are developing wireless directories. However, cell numbers will not be included in the list unless consent is given.

Also not covered by this law are bill collectors (either primary creditors or collection agencies.). These callers are, however, regulated by other laws, such as those limiting them to calling during "reasonable hours." The right to not be pestered by creditors is only protected for debtors who file for bankruptcy protection.

Additional help for locating who is calling from mystery numbers: