If you're truly free "all day", reply with a specific time and mark that time as "busy" on your calendar. If the recruiter hasn't suggested a specific time, they're likely waiting for you to settle on a time.
Not setting a time yourself can lead to a cycle of "you pick" "no, you pick" and though the indecision, the call never happens.
By suggesting a time, the recruiter can block the time on his calendar as well, so that he doesn't land in a situation of getting distracted/too busy during the course of the day and forgetting to call you altogether.
Leaving it open by telling the recruiter "I'm free all day" can cause a conflict with your schedule later in the day because now you have to leave all day open to take this call and more or less sit waiting by the phone. No going out shopping, making plans with friends, going to the gym, etc. until you get that phone call finished.