FRITZ!Box 6590 Cable Service - Knowledge Base
Do telephones connected to the FRITZ!Box work when it is not plugged in?
When FRITZ!Box is not plugged in, you cannot use the telephones connected to it to conduct calls.
If you have a landline and the telephone does not require its own power supply, then you can connect the telephone directly to your landline if there is a power outage. If you have an analog land line, the telephone must then be connected to the DSL splitter or "TAE socket 1". If you have an ISDN line, connect the telephone to the ISDN NT.
If you have an IP-based line, whether you can still conduct calls when there is a power outage depends on your provider.