Although the provided stats clearly show that the National Do Not Call Registry prevents most of telemarketers from calling your number, there are still numerous incidents when you receive such calls while being listed in the NDNCR. Some of the calls will surely be illegal and you can easily file a complaint about them. However, before filing anything, I'd like to encourage you to read the rest of the article, where you will learn a few exceptions from National Do Not Call Registry.
You also shouldn't bother complaining when you receive survey, debt collectors or non profit organization calls. These are not restricted by the NDNCR. There is also a regulation which let companies who have been in business relationship with you for over 18 months to call you without your consent. If you wish not to receive calls from such companies, you will need to ask them individually which should surely prevent them from calling you again.
You ought to remember how much time has passed from the moment you listed your number in the registry. There is a 31 days hold period during which telemarketers are still allowed to call you. After this time they should stop calling you. Very important thing is also checking if your number actually is listed in the registry. It can be simply done by visiting a verification page of FTC website.
If you believe that none of the above mentioned exceptions concerns your situation, you are free to file a complain. Filing it is very easy and can be done online or simply by calling 1-888-382-1222. You will be asked a few questions regarding the phone call which you think was unsolicited. FTC has also produced a simple movie about filing a complain:
The movie is not stricly about Do Not Call Registry complains but clearly shows the proccess of filing one.