They're scammers who leave voicemails telling you to call a different number, if you do call the number they tell you to they pose as a collection agency and try to steal your money.
It is a scam. If you call the number back, the message says it is a non-working phone. They leave a message to call "Barbara". They give a different phone number to call back, and it undoubtedly is forwarded to a fake phone. They are scammers trying to get money from you.
I actually answered this call, and it was a collection agent. I knew I had an overdue dentist bill. I had made a small payment, and agreed to call back the second week of JAN. I got a voicemail for a girl named Ashley, and it didnt sound very professional. Now I am wondering if I made a real payment or just gave money away........ I will definately investigate this.