Folks
A couple of technical points on mobile phones (from memory as the report I produced for someone else on this site is on my other computer)
Reception
As you know phones use UHF frequencies and transmit a digital signal. The ground antenna are set to be directional and a base will normally have an array of 3 aerials each covering 120 (with overlap) in the horizontal plane.
They are also set to "look down" so that little or no signal is transmitted upwards - which is why mobiles can be intermittent at altitude (I did work out from the specs how far from the transmitter you could get a signal at altitude - it was in excess of 40 miles)
This directional setting is the opposite to, say an, aircraft VHF aerial that broadcasts the signal through a 360deg plane.
Multiple site blocking
Can't happen, whan a phone signal is recieved by the system the ground operating system (computer) registers that phone to a particular site (the one with the highest signal).
As the phone moves it's signal is monitored and when the signal level from another base station becomes greater than the signal to the registered site the ground system then de-registers it from it's existing site & re-registers it to the new site known as handover.
Most systems are designed so that an individual phone cannot use two sites simultaniously. Though there is one system where this occurs briefly at handover only.
(We use a VHF PMR radio system at work that does the same, and can set the radio to actually display which site it is working through)
This again varies from aircraft where some national FIS services use mulltiple transmitters with offset frequencies to prevent interference if multiple sites are used.