Open Source PBX Does Not Mean Its Free
Open source is not free. Anyone can download the Asterisk PBX software and create a State-of-the-Art Virtual PBX phone system. However, the software download will be the last “free” thing you get. It takes a serious amount of time and expertise to actually make this type of system function at a level acceptable to any size business.
The following blog post by Search Unified Communications illustrates the problems experienced by a company that tried to implement an Asterisk based Open Source PBX and because of a multitude of problems ultimately decided to go with a Hosted VoIP solution.
Of all the points made by the author of the post, the most important point, in my opinion, was, that the business owner should concentrate on its core business and not dedicate resources to services others do better than you.