Was it this one:
Beautiful Loser by roxymissrose
Summary: If Jared knows one thing about life, it's that you just can't have it all. He's been Jensen's best friend since they were little kids; he's been Jensen's team mate—followed right along when he became a firefighter. He's been the guy Jensen turns to when the going gets rough, but the one thing he'll never be is the guy that Jensen loves.

The bottom of his world drops out when, on one of their calls, a fatal mistake on Jensen's part takes him out of Jared's life forever.

Only…there's this old ham radio Jared found in a box of Jensen's stuff, and when he turns it on, he's answered by a kid who strangely sounds an awful lot like Jensen…

This story is inspired by the movie Frequency