We know that Lalli tried to contact Onni in the dreaming. Onni's physical location was too far away by then, so he was unsuccessful, but Lalli definitely knew what he was doing so it can't be a one time only -thing. I'm therefore guessing this is how mages contact the base while they're on field - they aren't too far away as long as they stay somewhat within the same area of Finland.

Luonto might indeed have effect on this though, in which case the above example shows that you only need one mage with a bird shaped luonto to contact another one in the dreamworld magechat. Maybe Finns make sure the base always has some mage/s with bird luonto, so they can rely on having someone who can contact the team on the field.