Tails says "Please, Sonic. I'll tell you more when you get here!". There's no reason for him to be concerned about the security of the transmission (All the stuff Tails has planned at that moment is overt and easily guessable. Of course Knuckles being captured will cause Sonic to show up. What else would happen?), so it's obvious that Tails just wants Sonic there ASAP. Then you talk to a lumberjack. If you tell him no, Tails says "We have time to do this, Sonic. Let's help him!". Why?
Radios are crackly and sometimes things can be hard to hear or lost in transmission. Tails probably figured there was less danger of Sonic missing something if they talked face to face. Also, Sonic is impatient, and Tails probably wanted to be able to keep an eye on him and make sure he was paying attention while he explained the situation.
Because it's the right thing to do?...
Historians refer to the Nocturnus Echidna clan as the Fourth Great Civilization. What were Great Civilizations one through three?
The Gaia Temple Builders, the Babylons, and the Black Arms.
The Black Arms were never a vibilization on Earth, they were just aliens who passed by long enough to create Shadow. And the Gaia Temples are Sonic Unleashed territory, which came after. I'd say it was the Babylonians, the Knuckles Tribe(It's downplayed in Sonic Adventure, but Sonic 3&K and SA2 showed that the Knuckles tribe had a HUGE influence on the world.) and maybe the Human race.
So what's the status for the sequel? Is it never being made?
So at the end of the game, in Ix's chamber, all the chaos emeralds were collected and Sonic became super Sonic. My question is, how come Shadow, who was explicitly shown to be in the room, not become Super Shadow?