My problem was similar to yours, except as soon as I logged in I was getting the “503 service temporarily unavailable…” screen.
By trial and error I discovered that if I select my browser’s (was using Google Chrome) refresh button, I was taken to the FireRTC dial pad screen where I was able to make a call as usual.
When you get the “503 service temporarily unavailable…” screen, select your browser’s refresh button and see if it works for you…