You can only use Pakistani SIM card in United States if you have “International Roaming” activated on your SIM Card by the GSM service provider.
For example, if you are using Mobilink Sim card and want to go abroad, whatever package you are using Jazz (prepaid) or Indigo (Postpaid), you need to call the helpline to activate international roaming on your number which is free of cost.
When you will enter United States or any other country, as soon you will switch on your mobile phone the United States phone service will be activated automatically however if it doesn’t, you will have to go to setting, network setting and chose it manually to any United States Network (AT&T or T-mobile, depending upon if your GSM company has collaboration with it)
The service the very convenient however is very expensive, for making call to Pakistan it can cost you hundreds of Rupees. The better option is to call back home with a telephone booth located in airports and buying a prepaid SIM later on which is easily available for visitors. e.g. go to AT&T and tell them that you are a visitor and needs a SIM card, they will take your details and check passport and will issue a SIM card. You might find a booth for GSM company on airport as well but its not necessary.
While making a call from a phone booth you will need coins which you can easily get from any of the small shops in airports. Make sure you dial Pakistani number starting with 011-92-423-xxxx or 011-92-300 etc for cell phone. In USA you have to dial 011 to dial an international number, its 001 in Pakistan.