I recently purchased the SM5100B Arduino shield.
I got the board running with my Arduino , and this is what my terminal program spits out.
I have been trying to get this radio to connect to the AT&T cellular network. I should note this is a freshly activated GO Phone account, the SIM card works fine in 3 other phones. So I verified that the SIM card works on the AT&T network.
Starting SM5100B Communication…
+SIND: 1
+SIND: 10,“SM”,1,“FD”,1,“LD”,1,“MC”,1,“RC”,1,“ME”,1
+SIND: 3
+SIND: 0
+SIND: 10,“SM”,0,“FD”,0,“LD”,0,“MC”,0,“RC”,0,“ME”,0
+SIND: 7
You would think this would be “plug n prey” but I dont understand why this will not operate, and google/sparkfun forum dont return any solutions 
looks like RMA for me if it wont work…
Well as it always seems with whenever I try new technologies, first few attempts fail 
The solution was stupid AT&T “now days” bonds the IMEI code to the phone you registered the SIM card with, go figure I forget this from the late 90’s when cloning phones dough
So what I did wrong was give the customer rep. my IMEI number from the “go phone” I purchased, which I should of gave the IMEI number from the SM5100B it self. Once I phoned into customer service, the rep told me there not suposed to change these because once the account is activated it takes 180 days allow you change it? makes no sense, but she was very sweet (No really!) and she changed it to the correct IMEI, and taa daa! i saw the terminal program say “ring” so sweet! im online now…
I hope this helps anyone that has problems like this, pain in the but… but ohh well…
Sean-