No it's not normal, mobile data will deactivate when wifi is on, but it shouldn't drop the signal altogether. What software version/baseband are you on? Check for an update either with the LG tool, or in settings => system

 

v10d then I should think.

There are newer versions about, but yours may not be a Euro Open phone, hence not having it. You can update the baseband separately (I use 725, I had 405 back in April when I got the phone), although it will wipe your phone, which in itself could help

 

Not entirely sure how to check, I just know that mine is as it came SIM free from P4U. Not that it matters as I binned off the stock ROM as soon as I opened the box.

Anyhoo, with regards to warranty, you can easily switch back and forth between stock ROMs and basebands using LG's official SmartFlash tool, to it's safe. You can get your phone back to the software it was on when you bought it in a few minutes.

 

Yeah it should be Euro Open then.

 

I think this issue comes up when the signal is very low. I've got T-Mobile Comet. I was able to make and get call at office but not at home. So after some research and some tries I enabled 2G and I can make call. But if I disable 2g again it stops working. So I have to use 2G at home but since i have wireless at home that's OK.
So to fix it go to Settings-->Wireless & networks --> Mobile networks -->Use only 2G networks (enable)