Hey Scott,
Selling customers new lines so that they can get cheaper phones is a practice called "churning" and the people that did it will probably get in significant trouble if you tell AT&T. The reason they do it is because they get higher commission (much higher) and can therefore discount the phone more for the consumer.
AT&T uses a system which should allow you to just pop your SIM card into any compatible phone and have it work (as long as the phone is either programmed for AT&T or it's "unlocked" to work with any carrier).
As for porting the number over, yes you can do that. You'll need to call AT&T customer service and have them help you with that. They should be able to replace one of the existing contract lines with that number.
My last job was training T-Mobile sales reps, so I know a fair bit about this
)
Let me know if I can help further!