It's more important to look at the actual battery drain. The 37% you're seeing for Phone Idle is just the proportion of total battery usage that's being attributed to Phone Idle. If the battery is draining unusually quickly, then that might be significant, but if the battery is draining relatively normally, then that figure isn't surprising.
Do you recall spending any time recently in an area of poor cell reception ? That's often a reason why Phone Idle might contribute to abnormal battery drain.
It might be worthwhile to install AccuBattery to get more information.