user30675: I have a SIM808, connected via hardware serial to an Arduino Mega. Communication between the Arduino and the SIM808 is fine. However, the quality of the signal is listed as 0 all the time. (AT+CSQ) 
I've checked, and there is definitely 2G (and 3G) signal. Power is fine. SIM is unlocked, and works in other M2M equipment.
The network status lights blink rapidly (ie searching for a network) for about 5 seconds. Then it takes one long blink, then goes back to blinking rapidly.
The module is picking up the names of the available networks, yet does not seem to be able to register.
I'm using a prepaid data SIM card from Vodacom (South Africa).

user18821: I think you are not giving it enough power, SIM800 module needs 5v/2A stable power source in order to register to network. Try to use a desktop computer power supply it has 5v/2A output wire (refer to colors).

30675: <p>I have a SIM808, connected via hardware serial to an Arduino Mega. Communication between the Arduino and the SIM808 is fine. However, the quality of the signal is listed as 0 all the time. (AT+CSQ) </p>

<p>I've checked, and there is definitely 2G (and 3G) signal. Power is fine. SIM is unlocked, and works in other M2M equipment.</p>

<p>The network status lights blink rapidly (ie searching for a network) for about 5 seconds. Then it takes one long blink, then goes back to blinking rapidly.</p>

<p>The module is picking up the names of the available networks, yet does not seem to be able to register.</p>

<p>I'm using a prepaid data SIM card from Vodacom (South Africa).</p>
 18821: <p>I think you are not giving it enough power, SIM800 module needs 5v/2A stable power source in order to register to network. Try to use a desktop computer power supply it has 5v/2A output wire (refer to colors).</p>
