ADS-B transmitters may be purchased with or without the pilot changeable call sign feature. Most general aviation pilots use their aircraft registration (N number) as the call sign for ATC communications and filing flight plans. Commercial operators routinely use call signs that are not related to the aircraft registration but relate to the activity of the operation (United, Delta American, etc.). 14 CFR 91.227(d)(8) specifically addresses call sign requirements.
If you always use your plane's registration number for air traffic control communication, you can use an ADS-B transmitter with a non-changeable call sign. If you're not sure, use the questions below to determine what kind of equipment you need.
If you answer yes to any of the questions below, you need an ADS-B transmitter with a pilot-changeable call sign feature.
Do you use an ICAO call sign to communicate with air traffic control? (e.g. DAL123)
Do you use a local area call sign to communicate with air traffic control? (e.g. RDDL123)
Do you conduct commercial air taxi flights using a Tango November prefix for your ATC call sign?
Do you conduct emergency medical transportation flights using a Lima November prefix for your ATC call sign?
Do you conduct volunteer flights for organizations such as Angel Flight or Animal Rescue Flight and prefer to use their ICAO call sign (e.g. ARF223)?
Does your flight planning company provide a unique call sign for privacy reasons?