That is so unfair. Don't you hate when people think something that's totally not true? Really the best thing you can do when you talk to your manager about it is continue to calmly state that you never received a phone message or anything re: the calendar appt with S, and that you weren't aware that there were any issues between you and S.
Something really similar happened to me at an old job where this egotistical guy "tattled" on me to my boss about something that he made look like a negative thing when it really wasn't. I basically just calmly defended my actions to my boss and avoided the a-hole as much as possible in the future. I also learned something from that which has prevented similar situations from happening (at least, it has so far!) which is that I put EVERYTHING in writing and basically am super conscientious about that. So for example, if something like this was about to possibly happen again, you could send out an email to S and say, "Today, you asked me about some guy getting an appt on your calendar. I have not heard from the guy yet but will send out an email immediately after I do." (Unfortunately this still won't prevent a person from lying about leaving you a message when they haven't). Or an email could say, "This morning, you asked me for XYZ project and said it was urgent. I will have that to you by the end of the day." And then after you give him whatever it was at the end of the day, forward on that original email you sent with an update at the top. And so on. These emails act as proof (to ppl who are too ignorant to notice) that you really are paying attention to their requests and you are following up on them.