The Teleportation Circle spell (PHB pg. 282) states that in order for the spellcaster to "target" a remote teleportation circle he or she must know that remote circle’s unique sigil sequence. What if a player or party wants to use a permanent teleportation circle to teleport to another permanent circle? This use case is not covered – or am I missing it?
I am including permanent teleportation circles in my campaign as sort of a municipal service "if you can afford or justify it" to get from one city to the next (and it won’t always be safe, no). I have some ideas about how to set it up (as in how to have a "local spellcaster" set a destination for the portal) but I am curious if there is a method already defined elsewhere.
As pointed out by @enkryptor below players can learn the sigils for remote permanent teleportation circles and access them directly. In some locations (larger cities, for example) access to the circles could be closely guarded and controlled, while in other areas the circles may be abandoned (still functional) or dead.
If players were teleporting from the large city of Messantia to the even larger city of Tarantia it would be a lot like highly privileged (and probably expensive) public transit.
If players learned the sigil for the permanent teleportation circle in Tarantia and teleported to it "from the wild, without an appointment" they’d be met by armed guards and murder holes. In other places, who knows what they’ll find.
I think the answer to my original question is "this page intentionally left blank".
Working on it.