Number keeps calling - let answering machine pick up but no one leaves message.  What kind of idiots do we have out there?

This number has been calling my mothers home once a day sometimes twice for the past 7 days. I called and it rang for approx. 5 min. I called twice. No voicemail at moms house, nor when i called the number. Through searching via google and reverse phone number sites I've found that its a landline in Guelph, Ontario, that is with ISP Telecom.I'll be calling in the morning to try to get some answers!