Why? Cost mostly. Enterprise IT executives continually cited the opportunity to reduce PSTN access costs by eliminating costly PRI connections to local service providers with a single IP trunk (in some cases also using that trunk for WAN or ISP access).
Elimination of on-premises gateways was another key factor driving SIP trunking interest. Enterprises can eliminate both capital and support costs of PSTN gateways by moving them into the service provider network.
Finally, enterprises were enticed by the possibility of leveraging value added services such as the ability to create virtual local numbers in remote regions, have added control over inbound call routing, including leveraging such features as call routing based on time-of-day or call volumes, find-me-follow-me type services to enable call forwarding to mobile phones, and the ability to use SIP trunking services as part of a disaster recovery plan to enable rapid reconfiguration of in-bound calling rules in the event of a major outage.
So what's not to like about SIP trunking services? We heard several concerns about the security threats inherent in enabling a direct IP link between an enterprise telephony system and a service provider's network. Many IT architects consider the PSTN gateway to be sort of a firewall between their phone systems and the public Internet, shielding their IP-PBXs from attacks or SPIT (spam over IP telephony). The fear over potential attacks via SIP trunking services has spurred the growth of session-border-control and VOIP firewall offerings in the enterprise market from vendors such as Acme Packet, Borderware, Covergence, and Ingate. Fortunately most of the enterprises we've spoken with realize that leveraging SIP trunking means increased security vulnerabilities and have included security risk mitigation within their VOIP architectures.
The other major concern we heard was around service interoperability and availability. We heard a number of complaints about the lack of service availability, especially from global telecom providers. IT executives consistently told us that their major telecom partners are "dragging their feet" on delivering SIP trunking services, possibly due to the fear of cannibalizing existing PSTN access revenues, or due to SIP trunking taking a back-burner to development of other offerings or consolidation of recent acquisitions and mergers.
Interoperability continues to be a challenge as well. Support for VOIP platforms differs from vendor to vendor. While most support the market leaders such as Avaya, Cisco, and Nortel, support for smaller VOIP vendors varies. There is hope, though, for interoperability concerns. Back in the fall of 2007, the SIP Forum established a certification program for its "SIPconnect" (http://www.sipforum.org/sipconnect) initiative, a set of standards for SIP trunking providers and SIP-based IP-PBXs to ensure interoperability. But at this point in time only a small number of service providers--including AGN Networks (now Nectar Corp), Cbeyond, ISI, and PAETEC (via acquisition of McLeod USA), as well as system vendors including Acme Packet, Avaya, BroadSoft, Digium, Ingate--have achieved SIPconnect certification. We expect more providers and system vendors to meet certification requirements in the next year.
At this point in time enterprises should strongly evaluate the potential for SIP trunking services to reduce costs and expand service options, but also plan for mitigating potential security threats. Talk to your vendors about their interoperability plans, especially with regard to SIPconnect certification, and pressure those without SIP trunking service offerings to bring them to market.