sleep mode

When making a call the screen goes black (sleep mode?)...this is extremely irritating  particularly when calling operators and am asked to press 1 for this, 2 for that etc. then have to press top button in order to access screen before I can press asked for number.....is there any way to rectify this problem?  Apart from that, I am delighted with the Diamond...well, there are a couple of other things I haven't worked out yet, but that is probably my not yet used to it.