user: Question: What prefixes are priced at Pence per minute, fixed at all times with a premium of 3p/min?
Columns: ['Scheme', 'Tariff code', 'BTs retail price (regulated)', 'Approx premium', 'Prefixes']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Pence per call, fixed at all times', 'ff15', '5.10p/call', '7p/call', '0871, 0872']
SQL:

assistant: SELECT "Prefixes" FROM "Table" WHERE "Scheme" = 'Pence per minute, fixed at all times' AND "Approx premium" = '3p/min';