The cell tower's controller also looks at your signal strength. If it isn't high enough, it decides you aren't worth it's time because you are too far away and according to it's programming, you probably have another tower near you that can actually pick up the signal. I wasn't aware of them logging that, though, so I'm suprised by that (happily).

As for extending the antenna, that would improve your ability to receive, but transmitting, with the power levels of a cell phone, I don't know. I doubt it, because you are putting out less than a watt IIRC.