No offence gents, but is this really the best place to be discussing how we may or may not use technology to intercept phone calls?
Nothing been written on here that is not all over the web. The techniques used to make/break GSM encryption have been discussed at length on open fora since the launch of GSM.
A much better solution would be to use
encrypted VoIP - much more secure to deploy, and there's no trace in the local telco's records, as their switch does not do any of the routing. The IP packets won't reveal the number being called, if there is one. No numbers on Skype for example, which is also encrypted, though not the the strongest standard. VoIP calls can be made from many PDAs, some mobiles and all PCs of course.