When exactly is the "there is a teleporter" information said?
The Overwatch Wiki says that when Symmetra places it, someone on the opposing team will announce its existence. When someone on the enemy team actually spots the teleporter, they will announce that they have found it.
To slightly expand on the existing answers: There are three different cases when a voice line is triggered regarding the enemy portal.
At the same instant she places in on the ground, someone on your team will say there's a portal.
When a friendly (or YOU) spots the portal. Now this is interesting because even your hero can announce that they've found the portal, even though it may not be in your line of sight. I've had many instances where my hero would say they've found the portal even though I haven't spotted it with my own eyes. I'd start spinning around looking for it to destroy it.
The last voice line would be triggered the moment a friendly destroys the portal.