What's going on here? Several times lately when I've finished a phone conversation and try to touch the red hang-up bar to end the connection, the screen goes black. I touch the screen where I think the hang-up bar should be located but it does not disconnect. Turning the phone off and back on usually corrects this ... but not always. On those occasions removing and re-inserting the battery is the only way to correct the problem. What's happening and what can be done about it?