Is it an OT-800? I couldn't find a QT-800.
You check the IMEI by dialling *#06*
Providers can use the IMEI of this device along with the model number and network to generate an unlock code. I assume it's locked to the former T-mobile UK? If you bought it online, please bear in mind that there are a LOT of different networks worldwide called T-mobile eg. T-mobile USA. They are not the same network.
I've not used this site personally, but you could start by looking here:https://www.unlockbase.com/unlock-phone
Numerous other sites are suggested by the giffgaff unlockopaedia.
If your phone is locked to T-mobile UK and you bought it second hand, you can also call EE/T mobile on>**phone_number_removed** for a code. The fee is £8.99. There is also an online unlocking form, but you can only use this if you are currently a T-mobile customer and you bought the phone from them personally.