What is the command to determine whether the modem is registered on a network?

Using a terminal emulator such as HyperTerminal for Windows, use the AT command "AT+CREG?" to determine the status of the network registration.

The response from "AT+CREG?" has the following output format with the following meanings:
+CREG:<mode>,<stat>

<mode>
0 Disable network registration unsolicited result code (default)
1 Enable network registration code result code +CREG:<stat>
2 Enable network registration and location information unsolicited result code +CREG:
<stat>,<lac>,<ci>if there is a change of network cell.

<stat>
0 not registered, ME is not currently searching for a new operator.
1 registered, home network.
2 not registered, ME currently searching for a new operator to register to.
3 registration denied.
4 unknown.
5 registered, roaming.

For example, if you get a response of "+CREG: 0,0", then your device is not registered on the network. A possibility for this is that the SIM card is not installed, not fulling inserted, or the SIM socket is failing (in which case AT+CPIN? would report ERROR even with a SIM card fully inserted).

For more detailed descriptions of these commands, see our "AT Command Reference Guide for GPRS Wireless Modems" at:
http://www.multitech.com/en_US/DOCUMENTS/Families/MultiModemGPRS/manuals