When there is no active telephone connection, the FRITZ!Box extensions are supplied with approximately 35 volts DC and about 32 volts effective ring voltage. In Germany and the Netherlands, this is a sinusoidal AC voltage signal with a frequency of 25 Hz, and in Austria a frequency of 50 Hz with harmonic distortion of < 15%.

If there is an active telephone connection, the loop current is 20 mA. The ring voltage may be loaded with a maximum of 10 mA. The analog extension is supplied with a maximum load of 30 mA.