We have been experiencing the same fast busy issue for outbound calls since last week, and instances have been increasing as well.
It has impacted our business as we cannot call out to our clients.
RingCentral support has not been able to provide a solid response about this issue. The only thing we have been told is that they are reaching out to their underlying carrier.

We have determined that this is not site specific, as our team member calling from an RC app in his home computer also encountered the same problem.

We have been using RingCentral for over a year, and has only encountered this problem, a week or two ago.

This is systemic to the RC core code - and the only work around w've found is to the change the outgoing number on the dialpad (if you're allowed to) to a Direct Number,. The calls go through, but now that person has the direct line to the staff member - not good - when you want them to call back the regular company number.

833-247-7694 - that's the RC Premium support - call them and raise a fuss.

Forgot to say - that fast busy = the other carrier blocking the call, and showing 'Scam Risk' to the people you're calling. It's been three weeks now, and still RC states that there is 'nothing wrong on their side...'