Embedded in RouterRT-N16 embedded Voip server 
Videos demo  - Cybercallshop  Video demo Cybercallshop action on E300 linksys CyberhotelCyberhotel on RT-N16 Asus router

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Comdif Telecom is specialist on embedded Voip in router.
What are the advantages

Linksys E300 - Embedded sip server Comparing to local Sip server:

- Save server price
- Be sure if network and router is on, Sip server is on
- No more NAT failure

Comparing to Centrex:

- Save bandswitch, internal call use only Lan
- Can use local FXO Gateway for backup
- Keep your phone system working in case of Wan failure
- Free choice for Sip provider

In all cases this system is very cheap and easy for maintenance, all is Embedded on
Flash Usb key, just change the key with a copy to restore your PBX.
Embedded is also very low power consumption and is a very green device.E4200 embedded Asterisk server

Disadvantage is only that you are not on a powerfull server and use is only
for smal business.