I have seen a couple posts where people were not able to get a signal on site-a towers. Unforuntaly I'm pretty sure site-a is the only one that carrys ALL traffic all the time. I'm guessing it's the 'central' most site--always a radio affilated with it on all talkgroups.
For instance. Site-E-westside, if a phoenix unit was way out west towards transmitter, his radio would probably be affilated to that site so you would hear the traffic on the talkgroup he is on, on the site-e control channel. But still being able to hear him on Site-a, because other radios are affilated to site-a on that same talkgroup.
Once he comes closer to the site-a tower, site-e would no longer be 'active' and you would not hear him anymore on site-e.
Maybe someone should write a sticky on how the phoenix system works, and which sites are always active. I would but it's only speculation coming from me at this point.