WARNING[XXXX] app_dial.c: Unable to write frame

I keep seeing this warning message come up in my log file:
WARNING[XXXX] app_dial.c: Unable to write frame

I searched Google like a madman and have found nothing relevant.

Does anyone have ANY idea why, what it could mean or how to fix it?

Why do you think it is a problem? It might just be that the caller has hung up whilst listening to an in-band progress message, etc.