Default [Q] i727 shows good cell signal cannot receive and place calls

I have i727 (formerly on AT&T network now airvoice wireless which are pretty much the same)
which is unlocked and upgraded to Android Jelly Bean

Sporadically yet almost every day it at times
shows good cell signal but cannot receive and place calls

tried "booost signal" app
turning on/off airplane mode
various network modes selectable through *#*#4636#*#*

nothing helps!!

except restarting but even that sometimes needs to be repeated a couple of times

please help!