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One thing I would ask is how good is your internet provider? I have AT&T DSL at home that is very variable in speed and has some occasional reliability problems. If that was the level of reliability I could count on for business telephony needs, I would not feel very comfortable depending on a cloud based phone system. If you have a good ISP, this would be less of a concern, obviously.
I read a little about the Panasonic system and it sounds interesting, basically a mini SIP trunking system in a box. I wonder what the voice quality is like though? I use Google Voice sometimes, and while the quality is acceptable, it definately is inferior to a landline, at time breaking up or having that digitized voice quality. I assume that a system that you pay for has higher quality voice communication, but I don't know that for a fact.