Most IP-PBX users don’t realize that there is money to be saved by unbundling SIP trunks.   Jason Fanning at grnVoIP has written an informative article on this topic.  Among his tips is the following advice:

• Know the terminology. Most VoIP providers will sell you a VoIP line – which they commonly refer to as a SIP trunk. Although the definition of a SIP trunk varies from provider to provider, the underlying service can be broken down into four separate categories or sub-components: a local number with the capability to receive inbound calls (this is usually called a DID), outbound call termination (also called long distance or toll calling), caller name display for inbound calls (also called CNAM) and emergency 911 service. In the VoIP world, these are all separate services and can be purchased separately.
• Know what you are paying for. It is common for a VoIP service provider to sell the equivalent of one traditional POTS (plain old telephone service) line. That means that all of the components of the service are included in a single monthly charge, and are not broken out as line items. If your provider follows this model, get to know exactly what charges are included in your monthly bill. When you subtract the cost of the DID, CNAM and 911 components, your cost for outbound termination will probably be more than $.02 per minute, which is high.
• Understand what you need. Most IP PBX administrators can realize cost savings by purchasing VoIP services individually. This is because not every user of the system uses or needs each of the four sub-components in equal amounts. It is probabale that your users could share a single line with CNAM and emergency 911 service. This would allow you to reduce the cost of SIP trunks by 80% and purchase inbound and outbound calling services individually at better rates.
• Find wholesale VoIP providers that sell unbundled service. A simple Google search will let you find VoIP providers that allow you to purchase only the amounts that you need of each category. As a result, you can save money on your total monthly expenditure. For example, you can purchase DIDs from online providers such as DIDx.net. Similarly, outbound call termination, CNAM, toll-free origination and international termination can be purchased separately from other wholesale VoIP providers such as EZ Call.