You are correct, under OBR this is a ball. The mechanic for this is, "Time! That's a ball!" Turn to the PA announcer, official scorer, or the home dugout, and let them know what just happened.
Many umpires in my area think that with runners on base this is a balk. Even after reading the rule directly from the book during an association meeting I can't convince them otherwise and have given up trying. This is a pet peeve of mine... I can't take it!
