Project Management - PLM

Hello everyone,

I have a question in project schedule


If I have created a phase in a project, that phase has a duration 30 days, and tasks have been inserted under this phase, so the phase duration is controlled by durations of these tasks.

I want the phase duration ( 30 day) to be fixed, even tasks are inserted under the phase and the phase duration doesn't change by the tasks durations, so after tasks are inserted under this phase its duration remains the same (30 days) and if the tasks durations exceed 30 days, maybe wrong message appears ?