I have been a sprint customer for 6+ years and have had multiple phones and never had this issue until I got a mogul. I had the original stock rom and have tried only installing or leaving out certain apps on the phone thinking that it was some app causing the conflict and not waking the phone up when someone calls or sends me a text message. It kept doing it just about every day, but mostly in the afternoon and night. Then I realized that every time that this would happen, my batt was at 30-40 percent. So I started keeping an eye on the batt meter and recharging it if it reached the 40 percent mark. I have been doing this for several months now and this seems to have fixed the issue completely for me. Since then it has only failed to wake up on a few occasions - and these were all times when I was roaming a lot or using the device more than normal and didn't realize that the battery had drained to below 40 percent.
I have read and heard a lot about how you should not keep recharging batteries before they drain all the way down a lot because this tends to shorten the battery life, but I recently saw this article
http://www.pocketpcmag.com/blogs/index.php?cat=49
in which the opposite is found to be true. According to the experiments that were done, it was found that keeping a charge on the battery as often and as much as possible actually seemed to result in increased battery life. I don't know if this increases batt life or not, but I can say that I have not noticed any decrease in battery during normal use.
The next time that your phone fails to wake up check and see what your battery level is - for me I have found that as long as I make sure to not let the battery drop below 40 percent, it never fails to wake up anymore when someone calls or sends me a text message.
I hope this helps everyone that is having this issue
Jay