Each MAC has their own system/application for looking up what insurance a patient has. Mine is Connex, but other places I work for use Novitasphere, FCSO, etc,. You need to get set up with the portal for your particular MAC.
It seems that while the patient(s) is saying they have standard Medicare, they may actually be enrolled in a Medicare Advantage plan. This is not uncommon. You can also use Availity to find out if they have an MA plan instead of standard Medicare.