What companies provide VoIP, PBX phone systems for small businesses?

Nowadays, there are a lot of companies that provide VoIP, PBX phone system. Like Phone Power, they have VoIP for only $6.21 unlimited per month. In Talk, they are Excellent for small businesses for only $16.25 per month. Via talk provides worldwide free calling more than 60 countries and it is also unlimited for $99 per month.