New telephone system! Suggestions?

Nextiva is a cloud based VoIP phone system, hosting lots of small and medium sized businesses nationwide. the provides UNLIMITED business calling for only $19.95 a month!! Click here for the detail about this promotion!!

Hello young!

I am to insert a telephone system for a colleague. Now I would like to catch up a few suggestions with which telephone systems it good experiences made myself. Ability should it the following

1 S0 external/Voip able. Replace (target Fritz box 7170 of 1&1)
1 S0 internal
8 a/b connections
1 LAN connection not compellingly
Day/night circuit.

the moreover possible so quite everything is to be with PC (the whole CTI getns evenly ) to steer i.e.:
- FAX over PC (or a fax box thereby would be better the PC to be not on must)
- Integrating OFF (as above best integrates and with PC callably/adjustable)
- otherwise still so gimmicks like call list, sowas like Fritbox monitor, call bypass manager

I thought for the beginning would be a Auerswald 5020VOIP i.o. only the software (CTI as well as configuration) of those is so miserable (in my opinion)

Price plays first times none rolls should naturally no CISCO telephone system be

Thank you
mfg hamsters