When a call is placed over an ISDN line, the caller's number is usually sent over the D channel before a connection is even made (CLI – Calling Line Identifier).
Access to your own network is granted if the call number appears in the number list, or the caller is called back if the callback option is activated. If the device is set to provide security using the telephone number, any calls from remote stations with unknown numbers are denied access.
You can use call numbers as a security measure with any B-channel protocol (layers).
