ok the blind leading the blind. Have not read the ADSL tech docs for over 20 years now....
As others have said the phone needs 2 wires to provide the power for the phone. If one of the wires is broken, then the phone power is not there and does not work. Not 100% sure if its actually a one wire for send, one for receive now, but in any case if one wire is broken it does not work.
For broadband this is different. There are allocated frequencies for upload, download and also for normal voice talking. The filter you may use stops any broadband frequencies from being sent to the phone or vice versa. Both the upload and download signals are sent over both wires at the same time, which helps the broadband modem detect noise errors on the line and fix them, providing a more reliable signal.
As broadband broadcasts over both wires at the same time it can run with 1 wire, but it then reverts to comparing with the earth pin from the mains, which means it can no longer compare the signal between the 2 wires anymore. It therefore runs in a reduced slower mode, but still works.