An anonymous call is a call made with caller ID disabled, so the recipient cannot see the caller's phone number. This is done by dialing 184 before the phone number or setting the line to always-withhold. The recipient's display shows "Withheld."
Anonymous calls have legitimate uses. Privacy protection when you don't want to share your personal number, businesses hiding employee personal mobile numbers when calling, and medical facilities not wanting to reveal direct lines when returning patient calls. Conversely, to notify your number, dialing 186 before the number temporarily overrides the withhold setting for that single call.
However, anonymous calls are also frequently abused for nuisance calls, harassment, and vishing fraud, so settings that reject anonymous calls outright are available. NTT East/West's "Number Request" is a paid option that automatically plays a message asking anonymous callers to call again with their number displayed, then disconnects. Fees and eligibility depend on the type of line you subscribe to, so check with your provider.
Smartphones can also reject anonymous calls. On iPhone, a setting in the phone-related settings screen silences calls from numbers not in your contacts and sends them to voicemail (the label and its location vary by iOS version). On Android, methods vary by manufacturer and model, but blocking anonymous calls is possible through the phone app settings. Note that even when calling anonymously, the caller's number is recorded in the carrier's system. Calling anonymously therefore does not make the origin untraceable: investigative authorities may query carriers through legal procedures. Review detailed procedures in phone privacy settings for iOS and Android.