eFax forced upgrades?

I got an email from eFax informing me that I had exceeded the amount of incoming faxes allowed by the free eFax account I have. The limit is 20 or less per month.

I called efax to ask how could I receive faxes if I have never given out my free efax number?
Well they explained that some companies are in the business of harvesting fax numbers. They dial a prefix and keep dialing until they hear a fax tone. The company will then accumulate thousands of fax numbers and sell them to marketers.
Well – now I know.
But I do wonder, is efax investing in these companies (fax spamming) or doing it themselves to get customers to upgrade their free accounts to paid accounts?