I tried it once in a HS mixed doubles tournament semifinal where the opposing server had a hard waiter's tray serve that, while inconsistent, was enough to give my partner trouble when it did go in. And in my opinion, he was gaining a tremendous advantage by taking a large step into the court before making contact. This was NOT a pinkie toe on the line, this was a stride into the court. His second serve, by comparison, was a powder puff.
I said during the changeover, "Hey, you know you're foot faulting on nearly every serve? I'm letting you know I'm going to have to call it if it continues."
Next time around, it happens again, so I start calling it. Now it's an argument over the rules that I can't call it and we need to have an outside official calling it. And the only outside official available is their coach. Their coach watches for two points, which the opponent deliberately serves powder puff so as not to step over the line, and then their coach wanders away. He goes back to cheating, but I can no longer call it, even though it's blatant.
We win in 3 hard fought sets and lose in the finals.
As I have said in other threads, if it were social I would say it under the context of it's cheating and you need to be made aware you're doing it so you can do something about it before it hurts you in a tournament, but I won't call it in order to win. However, if it's blatant in a tournament, you get the same courtesy of a warning, but after that, you bet I am calling it.