Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_18536769_1 (call_sign VARCHAR, owner VARCHAR)

### Question ###
When vista radio is the owner how many call signs are there?

### Accurate SQL ###

SELECT COUNT(call_sign) FROM table_18536769_1 WHERE owner = "Vista Radio"