How to stop apparent ghost calls?

After work, I checked my phone and I had three missed calls for the same number. I called them back and they didn't answer. They texted me the next morning and asked me who I was. I told them and I asked them why they kept calling me. They claimed that they received mmultiple calls from my phone. This is untrue because 1. my phone was in my car and 2. I wasn't near my phone around the time they called me because I was already at work. I looked at my call log to see if my phone accodentally dialed that number but it didn't show up on the call log. I'm so confused right now. What could this be?