Its worse than that Jim... telecoms billing has a long established tradition of hole filling.
This is where when data for a period is lost (it happens a lot!) the billing management team take call data for a similar period and use this to "fill the hole" and hope the client(s) are too stupid to notice. If you catch them out they refund the call charges but you really need itemised billing and some nice software to do detect this.
And yes we do this as a service for *some* customers :-)
Also it is worth running standard telco reporting software on your phone logs - many years ago we were running said stats on our own phone system and caught a contractor working for us calling his family in SAfrica for hours instead of doing his work. His contract was not renewed and his reference from us was very carefully worded.
A report we ran for a large bank found one office phone made a long duration overseas call costing (then) thousands. The customer investigated and was happy with the "exception" report. Evidently the guy dealt with asian share prices and had the cleaner press auto dial on his phone every monday morning at 6am - until it connected - the speakerphone was left running an audio "ticker tape" until he finished his weeks work because if he hung up it was hell getting reconnected.
IMHO Telco billing is seriously fun stuff.