A sequence of digits dialed on a telephone handset or device, intended to initiate a diagnostic procedure, verifies the functionality of said device, and reports results. For example, a service technician or user might enter this specified number to check the speaker, microphone, or signal strength of a mobile communication tool.
This mechanism for built-in testing provides significant advantages, as it eliminates the need for external equipment or software during initial setup, troubleshooting, or repair processes. Historically, such sequences offered a quick, relatively low-cost alternative to more extensive hardware testing procedures, reducing both the skill level and time required for basic system validation. It has enabled manufacturers to ensure quality control, reduces product returns, and assists field technicians in resolving customer issues expediently.