Dumb question about contract with At&t

Excuse this because its probably a dumb question but if someone gets an Iphone without signing a contract and just pops in their SIM card, don't they also have to contact AT&T and add the iphone data plan service? Wouldn't doing something like that require a contract or an agreement of some form?

Or, is perfectly acceptable to get a new phone at any time and without their knowledge and add whatever plan that phone requires?

Answer is probably the latter but i want to make sure. Thanks