The goal call(X) will call the interpreter as if the system were given the goal X. Therefore X must be bound to a legal Prolog goal.
To handle a query which has multiple goals then:?- call((write(hello),nl)).Note that we cannot write call(write(hello),nl) as this would be taken to be a usage of call/2 with the first argument write(hello) and the second argument nl) ---and most systems do not have a call/2.
Note that call/1 is unusual in that its argument must be a legitimate Prolog goal. Also note that call(X) will be legal if and only if X is bound to a legal goal.