You have to sign up with the mobile to mobile plan for AT&T GoPhone.
With that said, if you do decide on that plan, keep in mind is a $1 daily access fee (only on days that you use the phone), then your normal minutes are 10 cents per minute, unless it's to an AT&T customer, then it's unlimited.
Also, you can add nights and weekends to your plan for $19.99 a month with the GoPhone plan. Personally, that's a waste to me, but *shrugs* whatever.