If you want to supply a parameter value once and use it multiple times within a query, put it in the FROM clause with an appropriate CAST.

SELECT  phonebook.* 
	FROM phonebook, (VALUES (CAST(? AS INT), CAST(? AS VARCHAR(255))))  
						AS Choice(choice, search_string)
	WHERE search_string = (case when choice = 1 then firstnme 
                 when choice=2 then lastname 
						when choice=3 then phonenumber end);

This query selects what the second parameter will be compared to based on the value in the first parameter. Putting the parameters in the FROM clause means that they need to be applied only once to the query, and you can give them names so that you can refer to them elsewhere in the query. In the example above, the first parameter is given the name choice, and the second parameter is given the name search_string.