snnet:
I wonder why they've so definitively announced they won't be replacing the UPS's then?
Because the number of customers who push for a resolution will be small.
I am moving our last landline to VoIP in the next few weeks. I'm not a wireless landline user, but I'd definitely be expecting a resolution if I were.
I note the applicable terms were updated on August 4 due to the recall (this is explicitly stated), but there is only a link to earlier terms is only for those prior to 22 March 2019. Both available versions state there is no service in the event of a power failure, however it's unclear how those terms were varied with the UPS installed. They clearly were, otherwise it wouldn't be necessarily to alter them.
Personally, I would take the approach that this is a power failure, caused by faulty hardware. The terms state the modem (sic) requires mains power but the service will not be available in the event of power failure. The USP guards against power failure. Without this protection, the service is materally different from what was offered.