Some telephones, depending on the brand and model, have the intercom function, which allows to begin communication by sending the signal. When the signal comes, the telephone set rings a signal different from the standard ring tone call. After that the telephone temporarily switches off from the telephone line and works as an intercom.
According to the various market evaluations, the IP telephony market is respected to beyond seven billion dollars annually including VoIP (Voice over IP) equipment, protocol stacks, and IP telephony services. As a major vendor of the market for IP telepho
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