Screen flashes on and off when using Voice mail iphone 4s

[dave69rock]

Hi
When using voice mail on the Iphone 4s the screen flashes on and off very quickly so when your asked to press a number or try to end the voice mail the screen flashes off so quick that you end up turning the phone off so you can dissconnect the call.

I’ve tried restoring the phone and re-booting

Cheers Dave