I don't think that I would ever call a footfault in a match. We play rec tennis- we don't have tennis pros watching our feet while working on our serves to inform us about footfaulting.
That said- I will NEVER understand someone who is called for a footfault and they throw a huge fit and deny that it is happening. When you are serving you are looking up at the ball- if the other team is looking at your feet then you just need to trust them that it is happening. I've only seen 3 people call footfaults in a league match. Every time it was a contentious match where the person calling it was likely just doing it to be a D-bag. Every time the person who was footfaulting denied it and it caused a massive fight.