You can designate certain numbers from ringing your phone. If such a caller was to call you, they will get a busy signal. This is not usual as most companies will send the caller to voicemail. However, as a blacklisted number, it is assumed that the account holder does not want to receive any communications from that particular number. Any RCS/SMS messages to the phone will be deleted before reaching the phone number.
A blacklist can be set on one of 2 levels. Level 1 will affect the numbers within that sub-account. Therefore, the secondary user will log into their account, and set the blacklist number which will only affect her numbers. The primary account can do that as well with their numbers, however – they can also do it on an account wide level so no phone number in the account will ring, and even secondary users are affected.