Don't get me wrong, I believe in learning things and challenges, but I personally try not to invest too much effort in any specific thing unless I know to a large degree of certainty that I'll be using this knowledge at least a few more times in the future to "pay back for the time".
If you're gonna be unlocking/hacking VoIP devices in the future and you're learning methods to save you time in the future, then I agree. But I get a sense that you've learned your lesson and will be getting an unlocked box "forever" next time..
I don't usually give up, but when I did a few times (after investing about 50-60% of max projected effort), it was the right thing to do. I was cutting my losses.