What does unlocked SIM card mean?
Can someone with experience using this shield give me some answers?
I've got this shield. It works perfectly: SMS & Data !
When you use the SIM on a phone, when you boot the phone up and if you need to key in the 4-pin thingy, then it's considered as locked.The command AT+CPIN=? will let you know whether it's locked or not. It's kinda scary if you wrongly write them in the firmware (after 3 times, it's locked).For a GSM module which I wrote the library for (not SIM900), I omitted this SIM pin code function fearing someone might wrongly enter them for 3 times!
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