I cant call US from my cell in Nigeria even though I purchased extra roaming credits before I traveled??
need more infromation, are you using the correct international dialing format with the correct country code? Is there a error message? IDD + country code + phone number, it can take up to a minute for the call to process through.
What do you mean by "international roaming credits"? Did you setup an international calling plan?
Be aware that if you have never activated it, international roaming can only be activated by calling customer service. It can't be done online, due to fraud reasons. Also not to be confused with international long distance (which is just for calling other countries while in the US). If you have never called to setup your account for international roaming, then its NOT activated (although you only need to do it once, and it lasts for years at least).
If the phone is able to connect to the local network, and you are simply getting an error recording while dialing, you may just not be dialing correctly, as wingrider01 pointed out. Its a bit counter-intuitive, but you need to dial as if you were dialing on a Nigerian cell phone (since you are using their network).
The dialing pattern to call US from Nigeria appears to be: 009-1-xxx-xxx-xxxx (where the x's are the area code and number of the person you are calling). But the following also works in most countries (hold zero to get the plus sign): +1-xxx-xxx-xxxx
Also be aware that there are situations where you may not be able to connect, even if you setup your account correctly. This includes technical issues on the AT&T network side, that you can't fix yourself (I've had this happen to me, the first time I travelled abroad with my AT&T phone). If this is the case, you will show no carrier, and your phone will act like there is no signal (although it may read reception bars, as the phone will still search for available networks, and may allow emergency calls).
You may have to resort to calling AT&T from a landline, but they can get that landline number and call you back right away, to minimize the landline cost. Call 1-916-842-4685.