I seem to remember some (single missions, possibly in the Mission Simulator) that had the description (...this is part X of the Templar campaign). These missions had voice briefings.
Having completed the game, I wanted to play these missions, but I couldn't find them anywhere.
I tried both Freespace 1 and Freespace 2. In Freespace 1 (regardless of whether the current campaign is the main one or Silent threat, I can't find it. Freespace 2 doesn't have any single missions.
Not only that, but there is no way to view the description of the missions in the Mission Simulator (only the briefing when it starts). So, I can't figure out where I could have seen the description "...this is part X of the Templar campaign".
Anyone know how to play these missions?

Dewd, if you want to play additional campaigns, I recommend you visit Hard Light Productions ( http://www.hard-light.net/forums/ ), the largest FreeSpace fan community there is. There are honestly dozens of usermade campaigns and mods available. Most of them are meant for FreeSpace 2 Open, but installing it over your FreeSpace 2 is easier than riding a bike. Do visit and experience the added wonders of FreeSpace!