In your scenario, the teleporters have been around forever, but only recently have people figured out how to use them (and presumably didn't realize what the teleporters were before then). So, I'm imagining Stonehenge-like ruins all over the globe, which people discovered just 15 years ago how to activate for teleportation. This would cause:
Technology arms race
Teleportation is an insanely powerful technology
Teleportation would be a world-changing technology. It could be used to enable powerful new types of power production, computers, medical devices, space travel, etc.. Crazy stuff would be possible even if people failed to figure out how to build new teleportation sites, and absolutely insane things would happen if people could figure it out.
World powers would swoop in to control it
Every major power would secure as many teleporters as it could, setting up black sites around each. They'd have their top researchers focused on trying to unlock the secrets of teleportation.
Except in the most unusual of circumstances, researchers would get full control over how the teleporters are used. For example, if a UFO had crashed at Area 51, you wouldn't expect to see American diplomats using it as a toy. Likewise, diplomats and travelers wouldn't get to use the teleporters except in the most exceptional of circumstances.
In short, each teleporter would have a military research facility pop up around it.
Wars
There might be a lot of wars. Reasons:
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World powers would want to control the teleporters and set up their own facilities around them. If a world power had too few, or wanted a monopoly, they might launch wars to take over teleporters. And since teleporters can apparently be gotten to pretty much instantly from any other, a single major world power might successfully launch a campaign to take over all of them, even if that world power is usually confined to one part of the globe.
2. World powers wouldn't want their enemies to master technology first, which could be an existential threat to them. So if they thought that an enemy was getting close, they might attack that enemy.
3. World powers would value top researchers and data as well as the actual teleporter sites. So, if they came to perceive an elite few researchers, they might try to take control of those researchers, whether through conquest, bribes, or mere kidnapping.
End game
After the initial shifts in power take place and research conclusions start to become known, some new status quo might be reached. For example, researchers might conclude that the teleportation sites have relatively limited abilities and they don't know how to reproduce them, in which case some of the conflicts might cool down. Though, the research wouldn't stop; since it'd be impossible to conclusively prove that teleportation technology can't be extended, the efforts to build on it would continue for generations until people started to feel reasonably confident that further efforts would be unlikely to succeed.
However, if research turns out to be successful, the world would have all sorts of new, insane technologies in just about every field. It'd be a crazy world of mind-bending possibilities, making it too complex to speculate on here. 693