ITAD Subscriber Numbers, or ISNs, provide a way of interconnecting VOIP PBXs by adding a number to the internal phone number of the target phone. The ITADInternet Assigned Numbers Authority, registry of "TRIP IP Telephony Administrative Domain (ITAD) Numbers"RFC 3219 number is added to the target phone number preceded by an asterisk. Therefore, only numbers and symbols which appear on a telephone keypad will be used.
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