While possible, that doesn't seem likely. Doesn't the phone have to do some sort of handshake with the tower to even show the bars? Or are you saying that the phone only has enough power to do the handshake?
My understanding is that when the phone is idle, bars are commonly measured based only on the quality and strength of the incoming signal. The outgoing signal might be stable enough for a handshake but not for a call.
Of course the other possible explanation for full bars but no calls is that the network is busy.
Of course the other possible explanation for full bars but no calls is that the network is busy.