It is a switchboard number Delaware Department of Health & Social Services - Herman Holloway campus in New Castle, Delaware. I was being contacted for a job interview, but anyone could be calling regarding any other form of DHSS matter.
I think it's a generic switchboard number that each dept is filtered through and is non-returnable. For confidentiality reasons they can't leave a message but will try to catch you at a time you will be available. So this number will show up as having called at various times of the day until they get in touch with you. If you are already attending you may know the number of that dept already. Call them on your appointment card number if you have one.