Roommate problems about guy I’m talking to

So I’ve been talking to this guy for a few weeks and I FaceTime him pretty much every night. It’s usually a conversation that involves my roommate and we talk and laugh and it’s all good fun. 2 weeks ago, she went to one of our friends rooms and said she would only be there for a few minutes. I facetimed this guy and he jokingly said that she could take her time and to text her that, so I did. After she had been gone for about an hour and a half, I texted and asked when she was coming back and all she said was that he wanted her to take her time. I told her it was a joke and all I got in response was ok. She came back after about 3 hours and I asked her if me facetiming her was the problem because I knew something was wrong. She said that she was just struggling to sleep well when she went to bed and I was still talking to him. This is understandable and I apologize and told her that she just needed to tell me when that happened and I would stop. After all I was under the impression she could sleep fine and this didn’t bother her. This week, she told me she was going to stay at a friends house to study for exams in silence since our residence building can be loud at times but I don’t find it that bad. I said it was alright but I wondered if that was the true reason. I talked to my best friend and she told me that when my roommate went over to our friends room for 3 hours, they told her that she had told them she thought I was having phone sex while I was facetiming this guy. I was really shocked and appalled, not only because this literally did not happen, but because she is telling our friends my business and I’m finding out through my best friend.

I really don’t know what to do in this situation. I need suggestions on how to bring this up when she gets back. I don’t want to make her feel uncomfortable but I was also not having phone sex. I also am really upset she’s telling people my business, especially since it’s just untrue.

Any help would be so appreciated.