32 ringtones. Range, max. 100 m. Waterproof transmitter with LED indicator, run by 1 x 12 V, A23 battery (supplied). Receiver with LED indicator and volume control, run by 3 x 1.5 V AA batteries (not supplied). Dimensions, transmitter 33 x 85 x 18 mm, receiver 154 x 95 x 33 mm. Only 21-951 has a built-in memory that enables selection of MP3-files as ringtones. USB cable and MP3 software supplied. Frequency: 433.92 MHz.