How would I go about making extra installment payments on my Next Smartphone?   [Subject edited for better exposure]

Can I pay ahead on my phone instalment plan without having to pay the while thing off at once?   [Subject edited for better exposure]

If I have a couple payments left on my OLD phone, can I pay it off after I trade it in and keep it and use the NEW phone on the Next plan? I have my new phone now, but I haven't mailed in my old one yet. After looking online, I only owe like $70 on it to pay it off. So, even though I have both phone

Dear friends or AT&T Customer Service, I have got an iPhone 6s from USA whose my sister brought to Brazil. In USA it worked well with AT&T sim card, but I am trying to unlock it and failed miserably all the time. When I try to use the AT&T webpage there is the message: "Looks like you haven't paid o