Carrie,
Portfolio is well known for calling people like this and they do not care if you owe or not. This is why this company's is constantly being sued for violations of the FDCPA and TCPA laws.  You can invest in a call blocking device, they run around 60,00 and you can block them, but your better off keeping a notebook of the calls along with any and all messages, also recording them if you speak with them. Check your states recording laws first, some states require you notify them they are being recorded.
State recording phone:
http://www.pimall.com/nais/n.tel.tape.law.html

Follow the steps in what you need to know and send them a cease & desist letter via the US mail, return receipt or certified.  

http://800notes.com/arts/Jb8EW-eDhQA/harassin ... ou-need-to-know
See Residents post here also:
http://800notes.com/forum/ta-34af6a034ba34b6/unending-collection-calls
Templates of Letters:
http://800notes.com/Phone.aspx/1-319-242-7350

http://www.ftc.gov/os/statutes/fdcpajump.shtm
http://www.consumerfinance.gov/askcfpb/search ...
http://www.consumer.ftc.gov/articles/pdf-0096-fair-credit-reporting-act.pdf
http://www.consumer.ftc.gov/articles/0149-debt-collection

To File complaints FDCPA/TCPA violations:
Your State Attorney General
Their State Attorney general
www.naag.org (you can find your state AG at this link).
https://www.ftccomplaintassistant.gov/#&panel1-1
http://www.consumerfinance.gov/complaint/
The BBB

If the calls continue, file those complaints and seek a FDCPA/TCPA attorney..