You have to go back through the Future machine to reach your future self in the repaired timeline. Make your way to "your" sky home in 2059. This is where you get your medallion, from your "future self".
(see the related solution question)
you leave and go to a different island
After you travel back to the future to get the Island Medallion, return to the present to leave the island (just as you did after completing the items), by using the Lab selection on the time device.
You leave the island the same way you returned to the future the second time, by using the Lab setting to return to the "present time", outside Pendulum's Lab.
Once you have your Island Medallion, simply use the time device again to go to Pendulum's Lab (12 o'clock).
Use the Lab setting on the time device, just as you did to get to the restored future.
After you get the Island Medallion, or anytime you want to leave the island, use the time device at the "LAB" setting, which takes you outside Pendulum's Lab in the "present". Then you can leave Main Street as usual in the blimp.
Once you have the island medallion, you can return to the present using the time device on the "Lab" setting (12 o'clock), as you can at any point during the game.
The oracle is in the building on the hill on Time Tangled Island.
On your first visit using the future machine, you get the time device from your future self, so that you can restore the items that are tangled in time. When you finish and get the Island Medallion, you still have the device. Set it to "Lab" and go back to Professor Pendulum's lab.
Time Tangled Island was the 3rd island in Poptropica and debuted in 2007, and was originally called Time Twisted Island.
Time Tangled Island, the third island of Poptropica, was released in July 2008.
There isn't a phone on Poptropica's Time Tangled Island. There is a cell phone on Nabooti Island.