a line is designated as internal if internal dialtone is present when selected.
e.g. Prime Lines, Personal O/G keys, Key Lines, Multicall Keys
Here is the description I found in the manual under Line Preference Programming.
When the user goes off–hook to dial, the system checks for an available internal line appearance. The system looks at the prime line first. If it is idle then it is selected. Otherwise the rest of the lines on the set are scanned for the first personal outgoing line that is idle, or a single appearance key or multicall line that is idle and is not an in-only line. If such a line is found then it is selected for dialing. Direct trunk select, CO line and private trunk lines are never automatically selected and key and multicall lines that have more than one appearance (in the system) are never selected. If a line is selected then the user is put dialing on that line. If there are no available lines then the user is prompted to select a line to dial on.
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