I've not seen the mobile service be "not connected" when coming back from idle. Unless you're in a weak coverage area.
When it wakes up, how many bars does it show? Ignore the color (whether it's white or green), as that's just a data connection. If you have bars, you should still be able to make/receive voice calls.
WiFi will disconnect when idle to save battery. When you wake it up, it reconnects, but the bars and WifI icons will be white until it's established a connection to Google. But this shouldn't affect voice calls, so long as you show a signal in the "bars".
In any case, I'd say turn the wifi off for a while and see if you still have the connection issues on wakeup. If so, might want to try swapping out the phone.
(Heck if you just picked the phone up, I'd go to the store and swap it anyway).