We're investigating a problem with incoming calls not working where they are configured to redirect (also-ring) to other number(s).
This looks to be affecting one of our call servers and not the other, so this explains why calls are working some of the time. Investigations continue.
We are moving calls off of the affected server, and we are starting to see calls working again.
Customers with problem with calls may need to get their VoIP phones to re-register - the easiest way is often vie the web interface of the VoIP phone, or by unplugging the power and plugging it back in again!