If the phone sustained any significant drops, then the lcd may have been affected at some point. This would absolutely cause the white lines on the display. Another reason why a screen produces white lines would be due to any amount of water damage recently or previously on the phone. While the phone may not have exhibited issues immediately after the water damage, it may have affected the display over time and is causing it to short out on you.
A couple of things to take into consideration before you get a replacement screen:
If the phone was not straight out of the box from your carrier or Apple, then it is most likely a refurbished product.
If the device sustained any liquid damage, then fixing the screen won't necessarily fix the problem.
I would take it to a repair shop and have them take a look at the device before proceeding with a screen repair just to make sure. My repair store takes in phones under diagnostics and applies the diagnostics to any repair costs if we do see a problem with the phone that is fixable. Make sure you go to a store that offers this option.
If you choose to do an at home repair, keep in mind that if the phone is water damaged, when you open it up, the phone could experience further issues after the repair.