The problem with this situation is that you really can't tell. You've got the wrong angle to tell if a player is stepping on or near a line that is 40+ feet from you and is perpendicular to your line of sight. Sure you can tell if he steps on the sideline, but not the division line. No matter what it looked like from your angle, you can't tell.
The coach may have been completely right. Just tell him, like you did, that you didn't see where the player was and that he'd have to talk to you partner if he wanted any more information.
I've seen partners call my sideline (as I was lead and they were trail) when the player was at least 1-2 feet inbounds...not often since most officials know better but this guy, for some reason, though I just missed it.
