We looked at numerous companies and selected MyFax as our partner (
http://www.myonlinetoolbox.com/Corpo.../Partners.aspx). The reason we like it is that is was very easy for us to interface our software for estimating, purchasing and invoicing into their service without our users even knowing it exists. For example, assume a contractor wants to fax a work order to a subcontractor (becuase they are not using the Internet yet). We bring up a window that allows them to print, email or fax. If they choose fax, then the data is converted to the MyFax platform and a fax is sent to the subcontractor without the contractor getting up to go fax using the fax machine.
This service is very good, and I can only see one thing that may be a warning to others. MyFax, as well as many of their competitors is deisgned for the small business owner and they are very strict on collections. They do not warn you if they plan on shutting you off for a missed payment so ensure to have your account paid up since they will just shut you off and wait for you to contact them. I always prepay based on the nature of our business, but that may not be appropriate for you. They may have even changed their billing practices and my comment may not even be warranted.
Very good company with good service in the remote chance you need service. Brian