Captain Marvel.
Amazing movie, you get to see Steve, Black Widow and Banner figuring out who the signal was sent to by Fury in that pager.
They realize that the pager stops working. And they turn around to see Carol Danvers standing right behind them and she goes.
"where's fury"
EDIT:
This is a post credit scene, and right after she says ''Where's Fury'', screen turns black and it says
"Captain Marvel will return in Avengers: End Game"