Lets start by asking this question a differant way…
Do you want to save money on the cost of a phone system, in installation and ongoing line rental costs as compared to a Traditional Lines based system?
Hosted – This is where you pay a provider to host either your extensions or numbers/Lines. they manage the hardware and software, which is often in a data centre with various backup systems in place, and all you do is buy an IP Phones such as a Yealink or Snom, enter the login details given to you by the provider and you start making calls over your broadband line.
Onsite Software IP PBX – An onsite IP PBX can either be a propriatary system or one comprising of Asterisk the leading and most adopted SIP based telephony platform, some products that use Asterisk are TrixBox, AskoziaPBX, AsteriskNOW.
If using an Asterisk based PBX Solution you will have to provide your own Server and download an ISO of the distribution and install it your self, you will also need to find a provider to send you calls via either IAX or SIP.
Hybrid VoIP and ISDN/FXO – This is a system that has a combination of both ISDN/FXO ports and SIP/IAX software/cards, this type of PBX is useful in cases where you need failover and redundancy and is typically a best of breeds system with cheap calls over voip and a back up to the ISDN/PSTN network.
As you can see there is a good variety to choose from when you look for a VoIP system, and an even greater number of small companies out there offering you SIP Trunks and Hosted systems, all in all though you can have the best voip system in the world but if you dont have a desent internet connection then you will get gaps in a phone conversation, calls braking up, one way speach or just no audio at all.
If you would like any further help on this please feel free to contact me.