also Choose an username..., i find your statement pretty confusing:
a while ago i found another conversation about this matter in a german forum, people speaking more in legal terms and it did not look as good as it seems to be in other countrys.
Just think about this: Let's use Motorola for the example.
Motorola lets you unlock the bootloader officially. You have to get a code from your phone and send it to Motorola, and they will send you another code. Then you have to input the code in fastboot and the bootloader gets unlocked.
What does this do? They get an ID of your phone, therefore they
know that your device has been unlocked. If you send it in for warranty repairs, they will look at your phone's ID, run it through their database, and bam, no more warranty for you.
Now ZTE: ZTE didn't give two craps about unlock, they mostly just let Fastboot unlock your phone with 'fastboot oem unlock' which is extremely nice. So, you can just go to fastboot and do that, and voila, your phone is unlocked. Need to send it in for repairs? You wipe everything from it, restore the stock system and recovery, then lock it with 'fastboot oem lock'. I believe there may be some trace that it was unlocked, but they won't look for it. They usually look for the 5 seconds screen, which tells you if you have the bootloader unlocked or not (Though you CAN get rid of it without locking, and I know about one or two people who managed to claim warranty with this trick). One person told us that he went to an EU ZTE store with his unlocked phone (with the 5 seconds screen) to claim warranty for something, and the store guy
asked HIM what that screen was about... He told him that it was because of unlocked bootloader and the guy just repaired it free of charge.
One other thing: Could you ask ZTE germany about this? Like, ask them if unlocking voids your warranty even though the law says not necessarily. Make that last part really clear. I'd like to see how they respond