Fickering black lines when on a phone call

Hi Guys,

Can you help with my query? I have an I Phone 4S, I cracked the screen several months ago, and used it for months like that with no problems.

I decided to get the screen replaced, so took it to a local shop to get it replaced. Since getting it fixed a new problem has appeared. When making or receiving a call,when the phone is away from my ear, I get these random black lines flickering over the screen. These lines are only present during a call, all other times, playing music, texting .watching videos etc everything is fine.

I took it back to the phone shop, they replaced the screen again, but the same problem is still there.

I have googled this fault, but there does not appear to be a solution.

Any ideas ?