Will an att locked phone work with the go phone plan?

I'm going to get a Note 4, and intend to use the prepaid monthly go phone plan from att, and I have a few questions before I actually buy it.

If I buy a phone that's still locked to att, will it still work with the go phone plan? Or does it have to be unlocked?

Will the note 4 work with the go phone plan?

Is there any way to buy the prepaid in more than just a month at a time? Could I just pay for a year?

I appreciate any responses I get.

Lisylue