Please check your Gigaset Base Station N300/N500 has a blue light on the front of it - the blue light should be solid. 

If there is no light - please check that the Base Station is connected to your router and re-boot the Base by pressing the button on the front of the box so that the blue light comes on.

Once the blue light is solid your phones will pick up an IP address and the error message will have gone - your phones will now be back online ready to make/accept calls.