Phone signal, but no phone?

I'm home alone with my dog, and I just heard a phone signal. It was very loud, and it lasted for about 3 seconds. i know for sure that my phone is turned off, because I don't use it very often. And my mother is at work in a town 40 minutes from my home, she would never leave her phone at home. So what I'm trying to say is that there is no phone in the house. And it was a signal that neither of us use. Are there ghosts wandering around or what?