To Delete a Subscribed Calendar on your iPhone or iPad please follow below steps.
- Open Settings menu on your iPhone or iPad
- Go to section Accounts & Passwords
- Find Subscribed Calendars RosterBuster (or the name assigned by you)
- Tap on the Subscribed Calendar RosterBuster
- Delete Account
This cleared the old subscribed calendar links on your iPhone or iPad. Now you can proceed with the iCloud Calendar integration. See here: https://rosterbuster.zendesk.com/hc/en-us/articles/115004690454-How-to-setup-iCloud-Calendar-live-integration