Call Forward is different than Do Not Disturb in this regard.
It is true that an internal call for a forwarded set will appear on an Intercom button at the forwarded set. However, this is possible only because the internal call is ringing at the destination set. If the call cannot ring at the destination set (because all its internal lines are already busy, because it is being used for programming or because there is no set connected), the call doesn't go through, the caller hears a tone (busy, overflow or overflow) and the caller sees a message on the display ("Busy", "Not in service" or "Not in service").
In short, whenever the caller hears a busy tone or an overflow tone, the internal call didn't appear on an Intercom key. In the opposite, whenever the callers hears ringback, the internal call appears on an Intercom key.