Adding students to speed dial

I am a student within the Davis School District. I was using teams and adding other students WITHIN my district to my speed dial list. Most if not all students it would add without problems, but a certain student it kept saying that they couldn't add them to the speed dial list. Why is this? Can this be fixed? If it doesn't let me add them to speed dial does that mean that I cannot call them using teams? Thank you for taking the time to answer these questions.