I keep having these phone numbers show calling without an area code?

My cell phone is set up to always show the area code with the telephone number, so the past two days I have been getting these calls that do NOT have an area code but beginning with the number 603-XXXX, always a different last four but the 603 being the same. I have not answered causious from this, and when I tried calling it back from a land line, shows disconnected number...

What can this be???

Some telemarketing trick is probably right.

I have found that some business phones which are toll free numbers or off site business phones (people work from home) use the phone number their company used when they set up the service. Then that phone is disconnected. And this happens with legitimate business calls for me.

Some telemarketing trick probably. I have gotten calls on my home phone (and I think my mobile) from area code 000-###-###).