This seems to be an automated computer dialing service.
They called to set up an appointment with them, and left a voicemail, but no human was on the phone.
Most annoying.

it is a medical messaging service.  They call either to remind you of a dr's appointment or let you know about medical test results.  They don't leave a message - they'll call you back when they can talk to you.  If you call them back, you'll get an automated message that explains all of this.   I recently went to Next Care urgent care - and suspect they use this service to notify patients of test results. Possibly other dr's use their service too, so that their own staff doesn't have to call to leave appointment reminders.