It is strangely difficult to listen to modern digital cellphone calls (it used to be very easy to listen to analog types). There's always at least two frequencies involved and it can hop around, both as it feels the need, and as you move between cell sites. Since the coordinating information for this is embedded in a moderately complicated digital bitstream, following it is a bind. This digital bitstream is subject to what might be called casual encryption (it's hardly military grade, but it's far from plaintext) and you must perform a cryptographic attack on the data you receive to extract telemetry and voice.
The technology to do it, in realtime, does exist. I would expect GCHQ to have it, but we couldn't get it to prove the point for a TV documentary I was involved with. It would suggest the Bad Guys had a level of signals intelligence capability which is not typical of what we've all been led to believe exists, so the Pakistan connection might follow.
I would first look at people physical security issues on people's handsets.
Phil