VoIP is nowhere near ready to replace POTS. If you rely on your phones in order to do business the money you save on IP phones won't come close to making up for the business you will lose.
Just think about it. How often do servers go down? How often do you have troubles logging into a server? How often do server applications crash? Now how often do you loose your regular phone service?
Servers are typically connected to a UPS that will give a 1 or 2 hour run time in case of a power outage. The phone company can generate their own power and keep running for days during a power outage.
If you can live with an 80% availability, if you aren't worried about viruses or hackers, then you just might want to consider VoIP
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