Your contact data (phone number) is not shown to your host in the first place.
A host can "unlock" and download this data. This is required when they need to provide their guestlists to the contact tracing authorities.
To begin with, Rumo won't notify you when your data has been unlocked.
But, if your data has been unlocked you can see it on your visits page.
14 days after your visit, the data will automatically be deleted. Forever.
There won't be any backups either, it will be gone.