Pen registers and trap-and-trace devices (PRTT) let the authorities see what numbers a phone is calling or is receiving calls from, as well as some other metadata, such as the length of the calls and some location info. These methods don't actually tap into the calls and reveal their contents, so the US government says that no probable cause or warrant is needed. Law enforcement (or intelligence agencies) just need to file a generic request with a district court, which then rubberstamps approval literally 100% of the time.
These applications are always sealed (including even the case numbers), and we the public know very little about them. But now, thanks to a three-year court proceeding initiated by the indefatigable Jason Leopold (and his attorney Jeffrey Light and his employer Vice News), a federal judge has done something unprecedented: released a list of PRTT requests. Specifically, all 235 such cases from DC District Court in 2012: