This is a number for UnitedHealth Care. They make calls to medicare clients for a variety of reasons. Some reasons could be to set up a time when an RN can come by and give them an overall health check which is a once a year benefit, I believe. They also call if you've had a recent hospital stay and they want to see how you are doing and how you felt your experience at the hospital was. It's a good place to complain about issues that you didn't like so they can make some calls and try to resolve those issues with the hospital.
To some this may be spam if you don't need or want these calls, but at least you know it's not a pedophile or complete spam like others have thought :)