K7.net.  No hidden charges, no limit. 

I too, have been an Onebox user for YEARS.  However, the greedy corporate monster, J2Fax bought out Onebox last year.  Although they promised no changes, they lied and now what was once UNLIMITED receipt of faxes now is billed by the page after you receive so many.

K7.net does not charge at all.  You just set it up and go.  We weaned everyone off the Onebox (btw, I was a BETA tester for onebox, so I used them since the 90's) fax number and the transition has been easy.

We keep the onebox for a general email box, voicemail and the occasional fax that slips through.  The main thing I liked about onebox is you could FAX OUT from their site.  So if you needed to email and fax docs to several receipients, you typed in the phone number and mail addresses at the same time and out they went.

K7.net also offer voice mail, like onebox does.  What I have found with voicemail/fax combo turned on, is many fax machines will disconnect if they hear a voice and not the fax "screetch".  So, we just omitted the voicemail so the K7.net is only fax. Works great and it is FREE FREE TOTALLY WONDERFULLY FREE.  I'd even pay them a monthly fee as long as they don't charge me per page.