My story with LG G4:
More than 1 year ago (Summer 2015) I bought LG G4 H815 (international version) on Amazon to be used in EU and Israel while visiting there and in USA with Freedompop SIM card. Almost immediately after 1 year warranty expiration the phone died. LG service center in Israel (I was visiting Israel at that moment) refused to repair the phone. So, after return to USA I started conversation with LG USA. LG agreed to replace the phone but there was a catch: LG USA does not have H815 in stock, so, LG suggested to send me either ATT version H810 or T-Mobile version (H811). I asked ATT version because it is fully compatible with EU and Israeli mobile operators (except LTE band 20 that sometimes used in EU, but not in Israel) and I also knew that ATT unlocks its phones for non-ATT customers (if phone is not stolen, paid in full etc.).
Unfortunately, when I submitted request via ATT web site, I got response
We're unable to approve your request because the IMEI number provided doesn't match a device that's currently on the AT&T network
ATT CS did not help : they asked me to go to ATT store (and ATT store pingponged me back to ATT Support), after this they redirected me to LG USA to request unlock code and LG USA responded "only service provider can request unlock code".
In next iteration, ATT answered that "IMEI not recognized by the system". After I pointed them to screenshot of their own unlock page where this IMEI recognized as "LG G4 H810", they found another reason not to unlock:
I do apologize it may have been sold to you at that time however at this time LG is no longer supporting the device and not allowing us to submit unlock codes or a request. Sorry for the inconvenience
I pointed them to serial number 610xxxxxxx (it means, the phone manufactured in October 2016), but got in response same copy-pasted answer:
I do apologize it may have been sold to you at that time however at this time LG is no longer supporting the device and not allowing us to submit unlock codes or a request. Sorry for the inconvenience
Immediately after this I accessed ebay, searched for "LG G4 unlock code" and got unlock code for my phone. It took about 1 hour after I paid for this service and the price was less $4.00