Let's start with what Dispel Magic says:
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a successful check, the spell ends.
Keeping that in mind, let's see what a magical effect is. According to Sage Advice:
Determining whether a game feature is
magical is straightforward. Ask yourself these questions
about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell
that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
If your answer to any of those questions is yes, the feature
Note that there are two types of magic in D&D. See the question from Sage Advice (Is the breath weapon of a dragon magical?) for an explanation.
So through the list, a permanent teleportation circle is not a magic item, is not a spell attack, and its description does not say it's magical. There are two options that seem plausible. First, "Is it a spell?". I'd argue that it is not a spell, since you cannot simply cast a spell and create one (with the exception of with Wish, but that's a separate issue). It can only be created as a result of casting 365 spells over the course of a year. The other is, "Is it fueled by the use of spell slots?". While its only use involves using spell slots, the portal that is opened is what's fueled. The permanent circle is simply a "marker" for the teleportation circle. Since it is none of these options, it is not a magical effect per se and cannot be dispelled with dispel magic.
A second argument, if the first didn't convince you: What does Teleportation Circle do? Let's look:
As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle [...] A shimmering portal opens within the circle you drew and remains open until the end of your next turn.
So the effect of Teleportation Circle is to create a portal to another point in space. While RAW doesn't explicitly state this, it is strongly implied by the following that a permanent circle is not a portal:
Many major temples, guilds, and other important places have permanent teleportation circles inscribed somewhere within their confines. Each such circle includes a unique sigil sequence - a string of magical runes arranged in a particular pattern.
I'd love an official source for this point though.
Back to the argument, since the effect of Teleportation Circle is a portal, and a permacircle is not a portal, I argue that the permacircle is not "the effect of a spell becoming permanent" which would warrant the answer that permanent means until dispelled.
One last point on the logical side: One may claim that the sage advice ruling was an oversight, and should include any effect that was created as a result of spells. To them I ask: Can you destroy the berries from Goodberry with Dispel Magic?
The following is my personal interpretation of the spell.
I believe that there's no magic holding a "Permanent Teleportation Circle" in place. I believe it to be more simply like a marker, perhaps a "dent" in the Weave so to speak. It doesn't teleport anything; rather, the normal Teleportation Circle does. A permanent circle simply serves as a marker for the spell to "lock on" to. The magic lasts 6 seconds each time the spell is used, but beyond that what is left is not a magical effect.