Preventing "pocket calls" - iOS 11 I know someone who is always "pocket calling" people, this happens because they use their phone (an iPhone 6s running iOS 11.4) they just put it back in their pocket and dont lock it. They then put their hands in their pocket and start pressing buttons by accident. 

Ive tried to get them to get into the habit of locking their phone before putting it in their pocket, but they haven't been able to. 

Is there a setting i could set for them where their phone auto locks after say 10 seconds instead of the current minimum 30 seconds. Or auto-locks if the light sensor notices its dark.
No, that is unfortunately not possible. Adjusting the habit would seem the only effective solution.