On a glow, gas or electric plane with a dedicated receiver battery, the Telemetry function on your receiver will report the voltage as well as received signal strength (RSSI) as these two functions are "built in". However, on an electric plane without a dedicated flight pack battery, the receiver will be powered by the BEC function of your ESC, therefore the voltage reported will be the BEC voltage delivered to the receiver. If you want to know the voltage of your motor battery on your electric plane you will need an additional sensor called a Lipo Voltage Sensor. See pic below.
Which type of plane are you wanting to set up with voltage reporting?