What is the Callsign with an Area of tamworth and frequency of 0 88.9?
CREATE TABLE "defunct_callsigns" (
  "callsign" text,
  "area_served" text,
  "frequency" text,
  "band" text,
  "on_air_id" text,
  "purpose" text
);
SELECT "callsign" FROM "defunct_callsigns" WHERE "area_served"='tamworth' AND "frequency"='0 88.9';