I can only guess that what they really meant was "позвоним всем с практикИ." The е/и ending in nouns is a common mistake for those who, let's just say, slept through their Russian classes. I don't want to sound judgmental though, mistakes happene, maybe it was just a rare slip up on their part there. I'm not sure why it is "позвоним" as it is like "we will call", but I guess that the context will cover that for you here. Всем с практики refers to all the people attending a certain practice class or work, again the context you have there will cover that for you.