LG V20 sold and new owner cannot unlock?

[ebay_v20]

I sold my LG V20 online and the new user cannot use the phone when they put their pre-paid sim card in?

I purchased the phone used myself as a refurb on amazon. I just popped my sim card in that I pulled from my Note 5 and it worked great!

The new user took it to T-Mobile and had them look at it and confirmed there were no locks on it?

I also called T-M tech support and they confirmed that phone is unlocked?

I am just wondering if I put an anti theft code on it when I had it and now cannot remember?

I did a factory reset when I sent it out, wouldn't this clear any security settings I may have done?

How can the user check for lock settings that I may have done?

Thanks for the help!

 

[mustang7757]

Hi, welcome to AC!
What carrier your on at the time you had the phone??

 

[B. Diddy]

Welcome to Android Central! It'd help to know the exact model number of the phone. If you don't remember it, ask the buyer to look in Settings>About Phone and let you know. There are at least 8 variants of that phone, and it's possible that this particular variant doesn't support enough of T-Mobile's bands. For example the H990T (for Mexico) supports none of T-Mobile's 4G bands:

https://www.gsmarena.com/lg_v20-8238.php#h990t
https://www.t-mobile.com/support/coverage/t-mobile-network