I bought a unlocked iphone 5 USA model and it can't use att gophone?

When I put my gophone sim card into iphone it shows sim failure and invalid sim. Please help

Added (1). My phone is Factory Unlocked/Never locked model. Works with all GSM phone companies (ATT, T-mobile, etc). Will NOT work with Sprint, Nextel, Verizon, MetroPCS, etc.

You can't use an iPhone as a go phone, att wouldnt allow it. If its unlocked go with straight talk, its cheaper since atts go phone plan suck for smart phones.