Right, have managed to get speak to one of my techie friends.
He confirms that phone companies are a huge spaghetti of different computer systems, each accessed by different sets of people. And different for each company.
? Call centre bods can access subscriber data, match names to phone nos and see billing databases (which effectively means all outgoing calls you paid for). In the old days, in some companies they could even see your voicemail PIN - most companies have now changed this after, ahem, incidents.
? Then there are network systems, which connect your calls and pass along texts. These work in real time and don't usually generate records. These are maintained by network engineers, who can tap into the live system and find out where a given phone is (pinging), and actually sit there reading the texts passing through system. 
So it's a much smaller group of people than call centre staff, but they have enormous capabilities. There's a story that some time back a engineer was caught installing software to take a copy of every text sent by some sleb's phone.
? Then there's the stuff that's kept under RIPA, so the police can do investigations. The police need signatures of senior officers to do this, and the phone companies have tiny teams of cleared staff to access the databases, and the access is fully audited. This includes all of the material call centre bods can see plus incoming call traffic data (who called you, not just who you called) and other bits and pieces.
If there's corruption, I'm guessing it's in the first two sections - call centres and network engineers. Why bother forging complicated paperwork or corrupting better-paid, highly accountable staff when you can get so much from the minimum wage cannon fodder?