SELECT "Callsign" FROM table_26828 WHERE "Coverage" = 'Dumaguete Central Visayas Region'
when coverage is dumaguete central visayas region, what is the callsign?
CREATE TABLE table_26828 (
    "Branding" text,
    "Callsign" text,
    "Frequency" text,
    "Power kW" text,
    "Coverage" text
)