I recently came to USA and had brought two Indian phones with me, but could not get any reception here in US for Airtel. I had heard that Airtel had ties with TMobile in US and that I was supposed to get network here. So, I dug around and found that the network was in a different band and could not be joined by both my Nokia and Samsung c3303.
Luckily, I had an AT&T goPhone lying around after I bought Samsung Galaxy S3, but it was locked to AT&T alone. I tried to insert my airtel sim, but it asked for Network lock pin.
After searching for a solution for a long time, I finally found the solution from
here.
SGH-A157 Network unlock procedure:
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Step 1: Remove sim card from phone and turn the phone on.
Step 2: Dial *2767*3855# This will initiate a Full reset for Samsung A157
(Note: According to one of the commenters below (Captain Greybeard), skipping this Step 2 will retain your IMEI number, and prevent the IMEI from becoming all Zeros. SO, DON'T DO THIS STEP. Only if Step 3 does not work, try Step2 and Step 3 together)
Step 3: Once the phone restarts, dial #*7337# This will remove sim lock and freeze
Step 4: Now insert Airtel Sim card. and restart the phone. You will get TMobile signal and will start receiving messages.
Update:
The above procedure would make the imei number all 0s in the phone. May be because of this, I was not able to use this phone in India with the same airtel sim card.
However, my AT&T sim card works fine in the US, and I have even called customer care to associate this phone to my sim card permanently. One thing to note is that AT&T will not accept 0000 as imei number. So, I provided the IMEI number printed below the battery in the back of the phone, and everything works fine now.
That's it! Enjoy!