In the case of Finish Line, a single 100Mbps SIP trunk at corporate headquarters in Indianapolis replaces seven PRI lines, Mitchell says. Inbound calls destined for branch offices all come in over that line and are routed to the corporate MPLS circuit connecting headquarters to the destination branch. The centralized nature of the call flow provides visibility into call patterns that just weren't available with PRIs, Mitchell says.
While the benefits are clear, businesses entering into SIP trunking should recognize that capital expenditures are necessary but can be countered with ongoing savings in operational expenses, Mitchell says.
And implementing SIP trunking is not simple. It requires preparation and planning and thinking out a lot of details, Grothe says. She recommends spending six months to a year of researching, designing and negotiating before trying to switch over.
- Align cutovers with the end of current trunk contracts to avoid costly penalties.
- Know exactly what services are required to avoid attempts by providers to sell more than what is needed.
- Make sure the current PBX supports SIP. If not, upgrade.
- Carefully inventory phone numbers. This is essential for a smooth cutover.
- Provide for reliable 911 services.
- Provide a redundant trunk that has a separate entry path into the building.
- Make sure the existing data infrastructure has enough capacity to handle voice traffic at high quality of service.
- Become familiar with session border controllers, devices that negotiate calls between networks and that provide security.
All providers are not equal, so customers have to shop carefully, Grothe says. A new Gartner report, "Critical Capabilities for U.S. Wireline Telecom Services," scores seven providers (AT&T, CenturyLink, Level 3, Sprint, TW Telecom, Verizon, Windstream, and XO Communications) on seven products, including SIP trunking. On a scale of 1 to 5, with 5 being the best, their SIP trunking scores ranged from 2.5 (TW Telecom, Windstream) to 4 (AT&T, Verizon).
Those are overall ratings, but service providers offer a range of different features, and customers need to decide which ones they need. For example, Sprint has a feature called Event Trunking, which allows simultaneous call capacity to jump to a higher specified level during a certain time period and then drop back down, says Tammy Kapec, a Sprint product marketing manager for SIP trunking. It offers a mobility option in which calls can be routed to workers' cellphones if the trunk goes down.
Some providers supply configuration guides on how to set up specific call management platforms to work with their SIP trunks. For example, XO has certified interoperability of its SIP trunks with equipment from more than 30 vendors, says Steve Carter, XO director of product management.
If possible, businesses should lab test services from multiple providers before contracting with one, Grothe says. Providers will balk for businesses smaller than 4,000 endpoints.
Even if comparison testing isn't available, businesses should test out SIP trunks in environments that exactly simulate real network conditions and traffic, says Mitchell.
He says it's essential to carefully plan the actual rollout of SIP trunking, and to establish a reasonable schedule. Initially Finish Line planned to roll out 20 stores per day, but cut it back to 10 because the transition was just too complex. Each site was getting an MPLS upgrade to T-1, new switches and routers and new phones, and doing all that properly without disrupting regular business required time.
Tim Greene covers Microsoft and unified communications for Network World and writes the Mostly Microsoft blog. Reach him at firstname.lastname@example.org and follow him on Twitter @Tim_Greene.
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