It took a couple of seasons before I consistenly could get numbers. While not having a number certainly does not mean that there was no foul, not having the number sure makes it tougher to sell. Getting numbers is a question of mental programming. Sure, in some situations you won't be able to get one due to your angle on the play, but those situations are rare.
I'd say that our philosophy is that you need a number for the fouls you call. However, if you do not, that does not void your call, but those situations should be few.
Never make one up though. That could get ugly...