Using Firefox, I just entered a 580 number and called my home telephone, it worked as expected.
Over a year ago I discovered that I wasn’t able to save a new telephone number in settings using Firefox, but when I logged into FireRTC using the Google Chrome browser, I could change and save telephone numbers and make calls as expected. After a month or two I was able to change numbers using Firefox, the problem straightened itself out.
Try opening FireRTC in a different browser.