Hi dear, how old is the baby? It's getting very common these days for people to show phones to baby to calm or quiten them down or to make them have food quickly. These things can be helpful for a while but it harms the baby in long run . You can try making them understand politely, but if still it doesn't work, you will have to be firm.

How old is the baby ?? Would suggest you request them not use mobile..you can have some games and music or other activities to engage the baby depending upon the age.