My husband was in Hospice Care and opted out, but now relapsed and may die. If he dies before Hospice is rejoined, are they called or 911?

If not on Hospice then 911 is called. Make sure if that is what happens you have DNR available to the EMTs. If not, they will try and resuscitate.

If on Hospice, then the Nurse is called. She proclaims time of death. Cleans the person up and calls the funeral home of record.

You should be able to get back under hospice care ASAP, and then you would call them. If you haven't started that ball rolling yet, then you would call 911.
You can call them today if you haven't already as they are available 24/7.
Praying for God's peace and comfort to be with you during this time.