Sometimes I will notice that my phone has no signal and if I hold the power button down it will tell me it is in airplane mode. To fix it I need to reboot the phone.
Clicking the airplane mode option once, waiting and clicking it again will fix it but often not. While in this mode wi-fi continues to work fine, just connectivity to the phone network is gone.
This is a pain because I could be missing calls etc while this is going on.
If the phone is put into airplane mode by me I get the little plane icon. This does not happen when it goes into airplane mode on its own.