user: Question: what is the callsign whtat uses 7BU?
Columns: ['Callsign', 'Area served', 'Frequency', 'Band', 'On-air ID', 'Purpose']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['7ABCFM', 'Tasmania', 'various', 'FM', 'ABC Classic FM', 'National']
SQL:

assistant: SELECT "Callsign" FROM "Table" WHERE "On-air ID" = '7BU';