If the line is accessed by a line pool, simply remove it from the pool. If the line actually appears on sets, you will need to go into dialing filters, build a filter to restrict ANY numbers. Then assign it to that line. I suggest adding an override to it to at least allow 911 to go out. People do strange things in an emergency and might get flustered. I would not want to be responsible for blocking any emergency call.
Another option, depending on your software version, If it is not a real old system, you can program the line to RING ONLY. The call will then come in on the intercom key and you will not have direct access to the line for outbound use.
It all depends on what vintage your system is.
MarvO said it