Eventually, the phone companies in North America did, in fact, replace all their hardware. They didn't do it to stop the phreaks, but simply as a matter of course while moving to fully digital switching systems. Unlike the crossbar switch, where the switching signals and voice were carried on the same lines, the new systems used separate signaling lines which phreaks could not access. This system is known as Common Channel Interoffice Signaling. Classic phreaking with the 2600 Hz tone continued to work in more remote locations into the 1980s, but was of little use in North America by the 1990s, MF Signalling is still used on various parts of the U.S. Network but is almost always out-of-band.