As far as the cell tower/service is concerned, it's really simple. When a call is routed to your cell, what's happening is a switchboard is pinging all the towers, the towers are replying that they have or have not seen your cell lately. The ones that have seen it lately then ping your phone. If your phone responds, then the call is forwarded to the phone. If your phone doesn't respond, the system reverts to other towers near the one where you were last "spotted". If it doesn't find your cell within a certain time frame (mere seconds), it routes the call to voicemail. (This is, in fact, part of the problem with cellular usage on aircraft. The towers have trouble handing off to each other fast enough, as well as trouble finding your cell based on it's last ping.) ETA: So, when you power down the phone, switch off the antenna, or leave the coverage area - it all looks the same to the system that simply cannot find your cell.
Imagine how complex a cell tower would have to be to continually send pings looking for any phones in it's area? Much more simple to have the phone ping the tower, the tower files that ping away in case a call comes in.
This is why sitting in the fab with spotty reception drains my battery 3x as fast as turning off the phone's antenna. If I leave my phone antenna on all night while I'm at work, my battery will be half discharged by the end of a single shift. However, if I turn the antenna off, I wind up charging my phone maybe 1-2x a week (no pun intended...and I don't spend much time talking on my phone...). This is because while I'm in spotty reception, the phone is pinging frantically (again, no pun intended) trying to find
some tower, for god's sake, any tower. My kingdom for a tower.
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