Secrecy of communications protects both communication content (conversation, email text) and metadata (who called whom, when, duration) from third-party access. Japan's Constitution Article 21(2) states "the secrecy of communications shall not be violated," reinforced by Telecommunications Business Act Article 4.
Protection scope is broad: call content, caller/receiver numbers, call times, durations, email recipients, and web browsing history. Carriers are prohibited from disclosing this information without legitimate reason. Article 179 of the Telecommunications Business Act sets the penalties: anyone who violates the secrecy of communications faces up to two years of imprisonment or a fine of up to one million yen, and a person engaged in the telecommunications business who does so faces up to three years or two million yen. Attempts are punishable too.
Exceptions include court-warranted law enforcement disclosure, carrier service operations (billing), and user consent. Wiretapping in a criminal investigation is governed by the Act on Wiretapping for Criminal Investigation, which lists the eligible offences in appended tables and requires an interception warrant issued by a judge. Drug and firearms cases are covered, and so are fraud, computer fraud, extortion and theft, but the latter group only when the offence is committed by several people acting in concert as a group with roles assigned in advance. Suspecting a lone offender, or simply having received a nuisance call, is not enough to intercept a line.
For nuisance call countermeasures, communication secrecy complicates spam filtering - carriers analyzing call content for auto-blocking may violate this right. That is why the services actually on offer work from the caller's number rather than the content of the call, and why the subscriber has to sign up before calls are rejected or the caller is displayed. Building the feature on the user's own consent keeps it clear of the secrecy problem. See call recording legal guide for details.