Hosted PBX is the notion that your service providers hosts your phone system for you. Then over an IP connection you are connected to that service provider. You don't have to manage anything. you simply plug ini your IP phone.
Who is familiar with this concept?

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I am familiar with the concept of hosted PBX.
Here are a couple VoIP services that provide hosted PBX:
Packet8
http://www.packet8.net/business_services/
RingCentral
http://www.ringcentral.com/features/virtual-pbx/overview.html
Also, InPhonex provides Unlimited USA/Canada Business VoIP service and will sell you the Linksys SPA-9000 IP-PBX adapter and IP-Phones for creating your own in-house small business IP-PBX system.
http://www.inphonex.com/services/business-unlimited.php
http://www.inphonex.com/products/voip-phones-devices.php