A good starting point to change a norm is to discuss it.
You could either discuss the norm directly or indirectly.
An example of a direct way is to ask team members how they feel about certain behavior, such as picking up phones during meetings.
An indirect way of discussing norms is by mentioning the consequence of a norm (longer meetings due to phone | call interruptions) followed by reflection on its causes.
However, monitor these discussions well because process conflict is lurking which can be harmful for team performance.