VoIP is VoIP.....
If the provider is giving you all the equipment (i.e. IP phones, Switch, etc..) then I have no real way of telling you the quality of service because it's going to depend on the equipment and setup.
If they are going with something like a all Cisco, or mixed Cisco/Avaya type solution than it should be very good. Again though it's going to depend on the bandwidth & number of lines you run on it.
As for how many lines?? How much $$$ you got? Their effectively is no limit (there is, but it's stupidly high).
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