Availability – for emergencies, such as a server

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Functionality – since the advent of the mobile phone, anyone receiving a call expects to be able to save a number against a caller name and see it when they call again. They can then greet their customers appropriately and choose to prioritise or reject other calls. The provider should also offer toll-free and premium rate numbers.
Flexibility – one of the key benefits of VoIP over traditional PSTN is the ability to quickly and easily ramp up based upon business needs. This may be as simple as a handful belgium telegram of new DIDs for new starters, or setting up a new office in another country. Ramping down should also be easy, with no penalties or lengthy contract terms.
crash or a power cut at your head office, you will need to set up backup routing, so that calls can still be answered. The VoIP origination provider will need to enable this.
Management – your internal systems need to be able to route an incoming call based on its data. This data is passed by the VoIP provider, so will need to be rich enough for your systems.
Price – it goes without saying that VoIP providers will compete heavily on price.
Outgoing calls
The industry term for outgoing calls is termination. The VoIP termination provider routes your outgoing call from one partner to the next until it is received by the person you called. This is completely independent of your DIDs. The criteria for selecting or assessing a VoIP termination provider are different from the origination criteria:

Quality – in this case, the VoIP provider’s partners can also affect quality. The provider may be using partners with low-quality service, resulting in poor sound quality.
Security – as you are paying for the cost of the call, it is essential that only legitimate calls are connected. The VoIP termination provider must allow you to restrict calls based upon factors such as your company IP address range, the country code of the recipient and cost type (eg no premium rate numbers).