International call progress tones for IP-PBX and VoIP telephony
Telephone Line Signalling
The guard tone is sent by the answering modem. It is used with V.22 & V.22bis modes to prevent the high-band data signal from interfering with the operation of billing apparatus in certain countries where in-band signalling is used, by ensuring that the in-band signalling tone at no time appears alone.
It is a single continuous tone with a frequency of either 1800 Hz or 550 Hz, sent at a level of -6dB or -3dB, respectively, below the level of the transmitted data signal power.
1800 Hz is used in the UK & some Commonwealth countries, while 550 Hz is used in some Scandinavian countries. Guard tones are not used in North America.