Peer To Peer Mobilage

Networks have different shapes. A simplified cell-phone network could be drawn as a central node, with a number of lines radiating out of it terminating at some regional nodes. The regional nodes would also have a set of lines radiating from each of them and terminating at a local transmitter. The local transmitter then has plenty of lines radiating from it, terminating at each of the mobile phones in its range.

There might be special higher-level connections for providing pipes across countries or to use certain facilities (satellites, routing/listening stations etc), but essentially, the network is a cascading spiderweb of nodage.

This shape provides direct and convenient access - except when you are
a) Not close enough to a transmitter to receive a signal
or
b) Local geographical features (being in a basement for example) impair the signal

It is possible to get boosters that pick up the signal in an area of good coverage, and transmit it out into areas of poor coverage - however, these are fairly rare devices.

With a little of the bandwidth available for playing video games, or browsing the internet, a mobile phone itself could become a participating (rather than a terminating) node in the network. This would mean that network coverage would spread out to wherever there were people - sure the signal might be a bit crappy sometimes, but there would be a signal in areas where currently there is not - as long as there are a number of people around to help carry the transmission.

Given a chain of mobiles all within range of one another, it might be possible to extend cell-phone coverage into remote areas otherwise completely cut off from the cell network.

In London, they say, you are never more than 4 feet from a rat - and the only thing more ubiquitous than a rat is a mobile phone (introducing small radio transmitters into rats by means of specially constructed foods is another idea for another day)