Assuming the sim card is active and works in another phone -(meaning u have an open account with an active cell phone number assigned already) -
then u need to look at other possible issues-some of these may seem simple, so skip over stuff u may have already checked-
Make sure the phone is fully charged, and make sure it is cut on.
If these 2 things are ok, when u place the sim card in the phone and cut it on, your carrier logo should come up, along with signal bars.
If this does not happen- could be any number of things-
Make sure no network issues in ur area- if there are, others with same service will have issues also- u can also chk this with ur carrier.
The phone may be locked even though it is supposed to be unlocked.
The phone may not operate on US frequencies - if it is a quadband, it should, but dual band, for example, if made for another country may not work here.
Contact carrier and have them update the imei number in the billing system- then power cycle (cut off for a couple of min, then cut back on) - this should force the device to register on ur carrier's network
Hopefully this will help get u started, if it does not, you will probably have to talk with ur carrier's tech support group to see if they can assist, and possibly to the manufacturer of the phone (company who made it- Motorola, Sony Ericsson, Nokia, etc.)
(The phone book, if stored on the sim, can be copied to or recognized by a device even if the device is not active on the network, but unless the device is active on the network, you will not be able to connect to signal to make a call.)