Penn Credit Corporation unknown, Harassing Harrisburg Pennsylvania

This company calls my dad during early morning late night during the day etc.  He is 90 years old and recently had two strokes.  I have had his pwer of Attorny and paid his bills for the last 12 years.  I finally got tired of the phone calls so I answered and they stated the usual questioning.  They said they were trying to collect an overdue debt and tht he needed to pay it today.  A red flag went up. I told them to mail me something to that effect and they asked for my address.  I told them they had it and hung up.  Who can I report them to so that someone will investigate their underhamded and possibly illegal activities.