Speaking on Deep Background, the Press Secretary whispered:
I'm told that most CALEA warrants only authorize a pen register, not an
actual tap. Pen registers are trivial to implement, since the provider's
software undoubtedly has an option to produce CDRs for billing or planning
purposes. Unfortunately this doesn't cover the case of purely P2P calls
which don't have a VoIP provider; if the suspect is using such software, the
only way to produce a pen register is with a tap.
Note that the requirements for a trap/trace aka pen register are
a fraction of those for a Title III intercept.
See A Maze of Twisty Little Statutes, All Alike: The Electronic Communications Privacy Act of 1986 (ECPA) for
at least an introduction..