These come from an entity called Jackson Med-Link in Jackson, TN. I assume they are similar to Alabama's TCC. I only ever hear a 3-digit DTMF string followed by the hi-lo alert tone, and sometimes a hospital replying "go ahead". Given this, I suspect a ground ambulance will request Jackson Med-Link to 'Ring down" a specific hospital, at which point they send the DTMF string to open up the specific hospital's HEAR radio, then use the hi-lo tone to get their attention. The ground ambulance can then give their patient report to the hospital. This is all assumption, based on what I can hear and how other areas have used the HEAR channel in the past.