Default Unlocking Bootloader

Hey Guys,

I'm not an expert at this. I recently took my rooted phone to a phone shop and the guy said he could unlock my phone for me for 10.00. He did it, but since then my phone restarts during calls.. I asked the guy to fix it and he said "because my phone is rooted, the kernel was damaged". Now, am i correct in thinking that the kernel can just be re-flashed along with a new rom? I have added custom roms to this phone before but now I cant unlock the boot loader. It won't work in flashtool, it gives me a "your phones bootloader cannot be unlocked". Any ideas?

Andy