if you've registered before there is no need to re-register - if you need to register a new number you'll have to do it on-line - this site won't allow a link so just google federal do not call and it will bring up the gov site -
"You can register your home or mobile phone for free.
After you register, other types of organizations may still call you, such as charities, political groups, debt collectors and surveys. To learn more, read our FAQs.
If you received an unwanted call after your number was on the National Registry for 31 days, report it to the FTC."
I went to the gov site - you can call them from the phone you want to register - "Add your phone number for free by calling 1-888-382-1222 from the phone you want to register