I'm using the softphone on a desktop computer and there are no error messages popping up, I just stopped being able to send and receive faxes a few days ago. Whenever I try to send something it just stays in the outbox and I haven't gotten any faxes since last week.
I experienced this same issue. Faxes just get stuck in the Outbox with no error. Faxes failing since 4/17/2015. I was using RingCentral Build 5 at the time. I upgraded to RingCentral Build 6.04.001.5, still unable to send faxes. I upgraded to Build 7.0.5 and I can now send faxes. I am not behind a firewall.