The tones are generated by a central office or a private branch exchange (PBX) to the calling party.
In Europe, some countries maintain national exceptions based on established older standards and have not fully adopted ETSI recommendations.
Increasingly, networks may opt to play their own domestic tones instead, making roaming seamless.
In some instances, the tones are entirely generated by the local network or even by the telephone itself; this is increasingly common on VoIP-based services.
The use of signalling protocols rather than audible tones means that a voice channel to the distant switch is unnecessary unless a call is connected.
The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist."