I've had a similar problem talking on the phone with friends. Many of my friends prefer to remain completely quiet unless they actually have something to say. I prefer an occasional "yeah, uh huh" just to make sure that the person is still listening or that the call hasn't been dropped." After talking for a few minutes without an audible response, I sometimes say "hey, would you mind grunting on occasion so I know the call hasn't been dropped?"

I don't know if it's socially optimal but it seems to work. (I only do this with friends who reasonably "get" me. I probably wouldn't recommend it with people you're not that close with)