You can find a dozen programs with a simple web search but your concern should be the connection used. MOST free or inexpensive software does not provide the stutter variance and latency is a major issue as a phone replcement.
NOTE: Your ISP or network will be more prone to problems and downtime that your telco, unless you have a weak phone service in your area but the POTS telephone switching network is MUCH more stable than the internet.
If you are referring to POWER outages a Computer based telephony system wouldn't work either.
Out of curiosity,why would you need a bckup for a phone system? I've seen basic POTS phone systems used as a backup for Computer telephony but not the other way around and have been involved in the telecom industry for many years now.
As for headsets, a decent noise cancelling feature is useful and comfort or fit is more important than most other specs.
Quality? With a free or low end software based system, voice quality and delay willfluctuate and will not be consistent anyway, this would take priority over the headset quality.
My advice, if you want to try it out, don't spend much money.
If you want a full blown VoIP system, spend LOTS of money but remember to save your existing phone service as a backup.
