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.
You aren't limited by existing mechanics when worldbuilding. As a DM, you are free to set your teleportation network as would be better for the story.
The Teleportation Circle spell itself suggests a wizard can learn additional sigil sequences during adventures when she meets the corresponding circles:
So if players want to use a permanent teleportation circle to teleport to another permanent circle they should probably be able to do that, providing they know the sigil sequence.