Received a fax today from this number, from Blue Shield of California. I emailed Blue Shield to validate its authenticity. The response stated it was a legitimate fax from them sent "by a third party vendor located in Canada". I AM a physician contracted with BSCa, so it is a reasonable fax.
I suspect it is a faxing (or other) distribution company that is hired to send faxes to the lists provided to them by whoever hires them, as in the case of Blue Shield of California, or, to use other databases for more general, unsolicited advertisements, etc.