The calls will never reach you
When you block a number on iPhone, the call from the number will ever reach you and you won’t get notified that this person was trying to call you.
The blocked person will hear one ring
If the blocked person called you, they will hear one ring before the call is dropped or transferred to voice mail.
They will be transferred to voice mail
The blocked person will be transferred to voice mail when they call you if you have voice mail set up.
The voice mail will be in your blocked folder
The voice mail the blocked person will leave will reach your blocked voice mails folder.
You will never receive their texts
The blocked person will never be able to send you text messages. The texts they will send will never get delivered to you.
They will think the text was sent
When a blocked person sends you a text, the message will be sent normally but they won’t see a delivered label. (See What happens when you block someone on Facebook?)
The iMessage will seem delivered
If the person sends you an iMessage, the message will seem to be delivered and will remain in blue color.
They won’t know you did
When you block someone on iPhone, they will have no way to make sure that you did, but they might guess it when they find their calls always reaching voice mail. (See What happens when you block someone on Whatsapp?)