[Sprint Samsung Galaxy S 4 Q&A, Help & Troubleshoot] CodenameDuchess:
After my AT&T Samsung Galaxy S4 screen cracked I figured that buying a new screen on eBay was the best course of action to take to fix the phone. While browsing for the best screen I came across a cheap listing of a Samsung Galaxy S4 already assembled screen (apparently the unit was DOA but the screen worked). Naively I thought buying this listing would be most convienent: all I had to do was switch out the internals and I would have a working phone and screen without having to take out the hairdryer.

It turns out, however, that I mistakenly bought the SGH-I720 model. There are several differences between the I720 model and the I1337 model. The external loud speaker from AT&T, for example, does not fit the I720 model. The digitizer touch component (located to the left of the battery) is constructed differently. The back casing (the plastic mold covering the internals) will not fit on the I720 model because of this digitzer component. Oh, and apparently I do not get any GSM service from my AT&T carrier.

The rest of the phone works perfectly though. Wifi, bluetooth, the camera -- etc. Everything else that was transfered over with the main motherboard works.

I suspect it's an antenna issue. Obviously, right? I do get some service though! I just have limited GSM connectivity. Is this because of a carrier lock? The key difference between the I720 and the I1337 is the bottom part of the phones (underneath the loud speaker). It appears that the I720 model has two antennas whereas my I1337 phone only has one. I'm assuming the I720 is capable of GSM connectivity (it says so on the product page) and therefore believe that the GSM antenna (on the right of the battery) should work. I also believe that it is properly working. Why else would I have signal (albeit weaker than I'd typically expect -- but that could be because I haven't fully constructed the phone and the case isn't on etc, etc.

So my question is: does anyone know how the carrier lock works? Does Sprint have it hardwired in the phone or is it purely software? If it is a software thing than I shouldn't have an issue -- I do, after all, have an AT&T phone in a Sprint body. Does anyone here with better technical expertise know why the bottom half of an I720 phone is simply incompatible with a motherboard from I1337? For testing purposes I connected the antenna from the motherboard in the I720 to the bottom half of the broken I1337 phone. I still get service but again -- limited connectivity and emergency calls only.

Any help? I'm thinking I should just cut my losses and buy a new screen and just manually replace the screen on the I1337 but I'm sitting on a working front end to a I720 and I really really want to make this work.

EDIT: I realize that it's L720 and not I720. Oops.

But here's an update: I moved back the motherboard from the L720 to the I1337 (where it came from originally). I just plugged in the screen from the L720 so that I could see what was going on (the I1337 screen is flat out broken). TURNS OUT that I don't get service period from the I1337 model! Which hopefully means that a) I'm not connecting something right or b) it's a really simple software issue that you guys know how to fix?

I did try calling 911 and I did hear a voice on the other side very briefly. I immediately hung up. So I have a feeling that my phone is connected to a network but isn't being properly recognized. Is there an emergency number that I could call to test my phone (that's obviously not 911)? I just want to hear a reocrded message or something that gives me a set of instructions...