At the beginning of my journey, if the contact has a phone number, I create a case (that will be sent to customer care). 2 days after, I'd like to check the status of that specific case and if it's in status A > exit or if it's in status B > send email. Is there a way to do this with a Decision Split?
Yes, that’s possible as long as you are synchronizing the Case object along with your Sales/Service Cloud data. If yes, then it should be already included in the data model and ready to use.
In the Decision split, choose “Contact Data” and find the Case object, then create a filter based on the status.
Contact Data is the most up-to-date data about the contact, which comes directly from Sales/Service Cloud and is synchronized on a time interval which is set in Contact Builder, usually it’s every 15 minutes.
Journey Data represents the state of a contact at the beginning of journey, it doesn’t change over time like Contact data.
You can read more here: Journey and Contact Data in Decision Splits
If you need more info on synchronizing data from Sales/Service Cloud, take a look here: https://trailhead.salesforce.com/en/content/learn/modules/marketing-cloud-contact-management/explore-synchronized-data-sources