On the issue of access to emergency services (which is somewhat different to the USO regulations over keeping copper), there are some interesting discussions and statistics to be found here:
http://stakeholders.ofcom.org.uk/binaries/consultations/superfast-broadband/summary/battery_condoc.pdfThe document is primarily focussed on backup-battery for fibre services, including the case where voice service is carried over fibre with no copper alternative.
If ever the USO regulations change to remove a guaranteed copper exchange line, towards a guaranteed IP-service (over which voice can be carried), then the consequent considerations for access to emergency service will probably match the ones in that document.
One snippet:
In 2009/10, there were 31 million triple-9 calls, of which 12 million came from landlines, of which 7 million were actually put through to an emergency service, of which 2 million were ultimately deemed to be urgent.
The maths behind power outages combined with different backup batteries, comes out with:
a 1 hour battery backup ... means that for around 99.95% of the time,
an emergency call is possible (or, put another way, for just over 4 hours in a year a
customer would find themselves unable to make a call due to a power outage).
For a two hour battery backup, this increases to 99.98%, and for a 4 hour battery this
increases to 99.99% (which translates to around 1 hour in a year that customer
would typically be unable to make a call as a result of a power cut).
We also note, and discuss later, that many customers have mobile phones, which greatly increases
their capability of contacting the emergency services in the event of a power failure.
Logically, the availability percentages of an N-hour backup battery apply equally for the exchange, cabinet, or ONT in the home. Or even mobile cells.