My Cox Home Phone is Dialing Numbers By Itself

I received a telemarketer call in the middle of the night last night.  I went to my telephone log this morning to see who it was so that I could block and report that number.  What I found was that at some point yesterday, my Cox telephone at home started calling a telephone number over and over again, more than 20 times in the past 24 hours.  Most of the time when these calls were made, there was no one at home, or we were sleeping,  especially between the hours of 11:30 pm last night and 6:30 am this morning.  I called Cox support and was told they'd never heard of such a thing before.  They sent a ping to the modem but said there wasn't anything else they could do.  Does anyone know what is happening, and how I can make this stop before I get in trouble for something I'm not even doing? Any help is greatly appreciated....