Straight Talk 30 plans only work with Straight Talk phones. You can port your number to a Straight Talk phone.
You may have seen people successfully implanting a SIM in an Iphone but it's against Terms of Service.
When they find you've done this you'll lose your service and your telephone # and will have no recourse. You simply can not legally use the Iphone with this service.
Yes, you can port your AT&T number (I did it), you just need to have your AT&T account number. Technically, you could use your iphone, but legally, you shouldn't, you should use their phones. Regarding billing, you can buy the plan cards every month or enroll Autorefill, with no obligation.