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.
VOIP PI Co., Limited. is a one of the leading asterisk telephony card manufacturer in china, with passion in supplying open souce hardware.
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