Where to get help for unrooted phone that won't boot after switching from original carrier? Recently my LG-P659 phone, originally from T-mobile, has had a lot of apps crashing or exiting for no reason, and today it can no longer get past the splash screen on boot-up (if it gets even that far).  When it gets to the T-mobile splash screen it ends up in an infinite loop at said splash, and sometimes it just ends up in a black screen.  

This phone was originally purchased in the USA by a family member, through T-mobile, but I am currently living in France for a few months and am using a sim card for a local carrier here called Free, using a month-to-month plan.  The phone was unlocked from the T-mobile network through a code provided to me by T-mobile.  It is unrooted.  The phone is no longer on an account with T-mobile, though the account it was originally associated to is still active (but I am not the owner of this account).

I attempted a hard factory reset, but all this results in is the error message "Secure Booting Error! Cause: boot certification verify".  I managed to get it to boot up once after that, when trying to get it to start in Safe Mode.  Everything was reset, but I made the (apparent) mistake of restarting the phone after getting some things updated and reconfigured, and now it's back to being stuck in the boot loop. Repeating the previous things has not been able to get me past the splash loop again.

So the question is: who am I supposed to call about this?  T-mobile, Free, the phone manufacturer, or someone else?  And do I have to call a French line or an American or what?  Making the call itself will be hard, seeing as my phone is dead, but I think I can manage to solve that issue.  And is there anything else I can reasonably try on my own, or should I just leave it as a brick for now?
T-mobile (toll free): 1-877-746-0909. 
T-mobile (direct): 1-505-998-3793.

I called T-mobile, and they said I had to contact the manufacturer.

LG (toll free, US): 1-800-793-8896

They also have a french version of the website, which you will go to by default when accessing lg.com if you are on a French ip.  You can otherwise access it by going to lg.com/fr . Conversely, you can go to the US version of the site via lg.com/us .  Either version supports a live chat function and has other relevant contact information.

That answers my question of who to contact for support, but I feel it is worthwhile to now note what options I appear to have.

After talking to the US support and French support, I don't really have any options for a quick repair.

I could send it in to LG in the US, which will take them 5-7 days after they receive it, but then they will only return the phone to an address within the US.  Taking into account the mailing times, the cost to get the phone there, and the cost for someone to send it back to me, then this gets a bit costly and slow.

They won't repair American phones outside of the US.  And to make things worse, my particular model isn't even available in France.

So the only reasonably quick solution to needing a phone is to buy a phone here in France.  What to do with my existing phone is a problem that's outside the scope of the question I initially asked, so I leave it at that.