You can simply register your number in the DNCR through Virgin Mobile app by tapping here. Alternatively, you can follow below steps;

1- If you're using an iOS device, tap on 'More' in the menu at the bottom of the home screen. For Android users, open up your side menu on the top left.
2- Tap on ‘Settings’.
3- Here you will see ‘Manage third party voice calls’. Enable the toggle and tap on confirm to add yur number to the Do Not Call Registry.