Hmmm had another read of your post, main O2 service centre?
Not sure about that lol
There are several different components in a mobile network, I will go with the 2G first as 3G has some differences.
First of all you have at the outer reach, the cells or Base Stations, these connect to a BSC or Base Station Controller, either by fibre/ethernet or 2mb connections.
Then you have something called an MSC or Mobile Switching Centre, you have several BSC's connected to one of these and anywhere between 1-200 cells per BSC.
You also have something called a HLR or Home Location Register, these store all the subscriber information, and every network has a lot of these.
When you make a call, the MSC interrogates the HLR to ensure your number (MSISDN) is on the network, when it finds you (hence you get a slight delay in the ringing), it passes your information onto the MSC which has something called a VLR or Visitor Location Register and your call is then set up and connected, this is basically a copy of your subsriber data, when your call ends the data is deleted.
There is probably a lot that the website doesnt tell you, like inter BSC handovers (handing over the call between cells when your moving), or BSC to BSC (on the same MSC) or between MSC's (so a BSC on one MSC handing your call over to another BSC on a different MSC), and you dont even notice it (most of the time unless the cell is really busy :mansurprised: )
Everything is normally connected by SDH (155mb circuit and above, also known as an STM-1, but you can get STM-4's, 16's & 64's) or PDH (2mb/34 or 45mb circuits).
Just to show you what your call would be on, its on 1 64kb circuit on a 2mb (2048kb) circuit, or one in 30 (there are 32 channels in total but 2 are used for signalling and syncronisation, depending on circuit type).
Hope thats not too confusing!