We're using a Grandstream HT502 to send from a FAX machine. It works sometimes. Fortunately, we don't send many FAXes, but instead scan to a PDF and email it.
We still have a POTS line that we use to receive FAXes (using HylaFax). That works well, but I'd like to get rid of the POTS line eventually. I have to be careful, however, because some customers still insist on FAXing purchase orders.