SIP Trunking in the Enterprise

Why consider a SIP Trunk?

1) Cost Savings. Local and long distance dialing charges are greatly reduced by using Voice over Internet Protocol (VoIP) delivered via SIP trunks, rather than legacy Public Switched Telephone Network (PSTN) via TDM trunks.

2) Flexibility. SIP trunks also allow for much greater flexibility. For example, in addition to just voice, Unified Communications (UC) can be delivered, including presence, video conferencing, file sharing and screen sharing. The benefits of a SIP trunk to today’s technology far outweigh any cost savings that adding a SIP trunk or even switching all together should be seriously considered.

Important considerations

Ease of Deployment

When considering a SIPTrunk provider, such as Sangoma’s SIPstation, consider how you will transition. Will you plan to purchase new end points (phones, door systems, pagers, headsets, etc.), will you work with soft phones that integrate into your unified communications system, such as Sangoma’s FreePBX Phone Systems or will you add a VoIP Gateway to add analog endpoints to your SIP trunk and possibly even provide PSTN failover?


You should prioritize a  provider that monitors the system in real-time and is proactive about possible issues on the network. Call access control is another important service. This technique guarantees delivery to the VoIP user at an extremely low latency.


VoIP has many benefits. Security is not one of them. Using VoIP over a public medium such as the internet does open both the Internet Telephone Service Provider (ITSP) network and the corporate network to vulnerabilities that must be properly addressed. Just as the firewall protects the data network, a Session Border Controller, is required to protect both the data and voice network when VoIP is integrated into the system.



ITSP and corporate networks protected by SBCs

The example shown in Figure 1 illustrates a legacy PBX on the corporate premises which has been converted to use SIP trunks rather than TDM trunks.  In this example, the SIP trunks are provided by the ITSP and delivered over the internet.  A Vega gateway converts between SIP and the TDM interface used by the PBX.  An SBC guards against toll fraud and navigates across the firewall. It also protects the corporate network.

The ITSP protects its network with a carrier class Session Border Controller (SBC) that is designed to handle the high call volumes that the carrier will experience and provide the High Availability (HA) features required for carrier operations. The corporate network is protected by a Vega eSBC (enterprise SBC) which is sized to handle moderate call volumes.  Both SBCs provide the same functionality, including prevention of toll fraud, denial of service and eavesdropping.  They enable VoIP traffic to navigate firewalls and ensure interoperability between different SIP implementations.