What is the difference between CALL and EXEC in T-SQL?

Consider:

CREATE PROCEDURE LowerCityDiscounts @city VARCHAR(45), @decrease DECIMAL(10,2) AS
BEGIN
    BEGIN TRANSACTION;
    UPDATE Customers SET discnt = discnt - @decrease
    WHERE Customers.city = @city;

    UPDATE Customers SET discnt = 0
    WHERE Customers.city = @city AND discnt < 0
    COMMIT;
END;

I tried to call this procedure with:

CALL LowerCityDiscounts 'Cleveland', 5;

but this only produces

Msg 102, Level 15, State 1, Line 1
Incorrect syntax near 'Cleveland'.

Yet, if I change things to

EXEC LowerCityDiscounts 'Cleveland', 5;

everything works fine. This despite that the documentation stating that call is the right syntax.

Why does EXEC work when CALL does not?