iphone 4 - restarts when call is received

Hi,

I know iphone4 is pretty old as we speak however it was my lucky one :-)

It was working perfectly fine until yesterday however now it switches off when we receive calls. it works perfectly when we make calls but while receiving it vibrates and switches off.Able to receive calls when its powered

Few things I noticed.

1. Battery charge was not empty and it was full

2. IOS is up to date

3. Done reset (Pressing home and power key simultaneously)

4. Restored to factory settings

5. Switched off 3G, WIFI etc

6. Able to make calls

Your help is much appreciated if you suggest me few ideas to fix it

Many Thanks