ATT go phone sim won't work in unlocked phone

Hello all, any advice on my issue below?

I've recently moved to the US and have an unlocked HTC Touch HD handset.

I needed a prepay sim to go in it so chose an ATT go phone. For some reason this sim isn't recognised in my unlocked handset. Any ideas why? The handset is definitely unlocked as a friends t-mobile sim works in it OK.

Is there something special about these go phone sims that might be causing it to not work?

If I can't get this sorted is there any other prepaid GSM sim I can use which comes with a data plan?

Thanks for your help.

D

ive used gophones regularly in the past

im pretty sure they dont work with regular phones that arent specifically gophones, and i know for a fact gophone sim cards/telephone numbers wont work with a regular contract or unlocked phone (you might be able to port a number over for a fee).

its just the nature of how gophone and regular phones dont mix and match, a lot of it has to do with the fact people used to buy the gophones in bulk for the cheap free sim that also came with the phone plus the phone itself, then turn around and use that sim in a regular phone without paying activation fees or anything (least thats what the guy at my local store said)

You may have purchased a GoPhone kit with $15.00 credit included. This combination locks the SIM card to that particular phone for 120 days. You should purchase an older kit from a retailer to get an unrestricted SIM card.

Will a ATT sim card work in any unlock Att phone?

++

Its been extended to 180days