If you are a service provider interested in providing VoIP to your customers, you essentially have two options: offer them premise-based equipment or provide them with a hosted service. Traditionally, enterprises preferred premise-based equipment because of the control and possible long-term cost savings that such an arrangement afforded them.
It is important to emphasize that, for enterprises where the ability to quickly modify telephony configurations is a mission-critical process, control over the telephony system should not be outsourced. However, for businesses that don't depend on the ability to quickly manage and configure their telephony system, the right hosted solution can offer advantages over traditional premise-based PBX.
Hosted service consists of the following feature set.

• Provides IP telephony service for SMB.
• Value added service for ISP, ASP and VPN providers.
• Class 5 IP-PBX for service provider or enterprise.
• No KSU at user’s premises.
• IP base Centrex service.