How to stop sudden onslaught of unsolicited calls to my iPhone?

I don't know what happened. I haven't signed up for any new service or done anything different, but about a week ago, all of a sudden, I started getting a bunch of unsolicited calls to my iPhone. Here's a screenshot:

Those calls come at any time of day. One was at 5 AM. Some ring maybe once or twice and somehow manage to leave a voicemail. Last voicemail was something about improving my credit.

Can someone explain why did it start happening all of a sudden? And is there any way to stop it?

PS. Before I'd get, maybe one unsolicited call a month, if I'm "lucky."

<Image Edited by Host to Remove Personal Information>