Pocket PC Phone Edition - Phone Problem - No ring

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I have a Pocket PC phone Edition with T-Mobile. It is recently upgarded to
Pocket PC 2003. But I have a problem:
1. If the phone is not in use for 20 minutes, then looks like the phone is
sleeping. That means, if there is a comming phone call, the phone does not
ring.
Do you know what happens?

I have to turn it on then it works again. But it is going back to the sleep
mode if it is not in use in 20 minutes.

Thanks,
Alan.