Assuming - and it's merely an assumption, but I'll see what people who know about such things say - that the Taliban don't have proper GSM interception equipment, then the most effective, cheap and reliable way to monitor and interfere with mobile phone calls is elsewhere on the network, as has been said. Tapping microwave links between base stations is usually the best way, but if you've got a pal in the network operator who'll let you bung some kit inside the switch you can do what you like.
Also see
http://en.wikipedia.org/wiki/Greek_t...case_2004-2005
There will be two ways to militate against this. Ban mobile phones, or run your own GSM network and maintain appropriate security -- in other words, see mobiles as part of your tactical communications strategy, and deploy suitably modified commercial off-the-shelf kit to support it. Operators do this anyway - they tow a COW (Cell On Wheels) into a field, pump up the mast, point a dish at a satellite and there's your infrastructure.
Perhaps Blandford Forum could have a word with Nokia.
R