This content is part of the Essential Guide: The basics of SIP trunking explained

Migrating from PSTN to SIP trunking: A primer

Replacing traditional voice circuits with SIP trunking offers cost benefits, but success requires the right SIP trunking provider and collaboration between network and UC teams.

SIP trunking offers enterprises a number of benefits over traditional analog or digital PRI connections. By carrying...

voice traffic over an Internet link, an organization can lower its costs. Migrating from PSTN to SIP trunks, however, involves more than simply adding a few more voice channels. UC managers must select a provider that meets the voice and unified communications needs of the enterprise and has the ability to collaborate with the networking team to ensure a secure and quality connection to Internet-based VoIP services.

Choosing a SIP trunking provider

Unlike the traditional voice services market, incumbent local exchange carriers (ILECs) and the competitive local exchange carriers (CLECs) that run on the incumbents’ copper are not the only options for enterprises that are shopping for a SIP trunking provider. A number of Internet service providers offer SIP trunking services alongside their broadband access offerings, offering an alternative to ILECs and CLECs.

Not all SIP trunks are alike. SIP is an industry standard protocol, but actual implementation and interoperability among IP telephony vendors and SIP trunking providers can vary. Before committing to a particular SIP trunking provider, a UC manager should ensure that its service has been tested with the the enterprise's installed PBX or IP telephony solution. Interoperability testing between SIP trunking service providers and the enterprise’s chosen UC platform will ensure a smooth deployment and should be high on the list of priorities in the evaluation process.

When evaluating SIP trunking services and providers, UC managers should also consult with their enterprise's networking team on the service-level agreements (SLA). Standard SLAs vary greatly among both providers and the type of services that an enterprise subscribes to. Unlike traditional analog or digital line service providers, a SIP trunking provider may not control any aspect of the connection that brings its service into an organization once it leaves the provider's own network. As such, an SLA between provider and subscriber will only apply to the components the provider can control., for example, denotes three different levels of support for its SIP trunking offerings, based on how much reach the provider has into the customer network. If the customer is also a subscriber of’s Internet service and has purchased a router from the vendor, the provider will guarantee the quality of service for the SIP trunks all the way to the customer premise. However, if the customer uses a third-party Internet access and its own router,’s support ends at its own network.

Some UC vendors simplified the process for customers by packaging their UC products with SIP trunking services from service provider partners. These branded solutions offer small- and medium-sized enterprises one stop shopping for their UC products.

“Many of our Mitel customers are going directly through Mitel for their SIP trunking,” said Tim McNicoll, technology solution sales manager at Wisconsin-based Heartland Business Systems, a value-added reseller of unified communications solutions.

Getting connected to SIP trunks: From PSTN to SIP  

A conversion from PSTN lines to SIP trunking services shares many of the same steps of any other telecom migration. SIP trunking providers support number portability, so an organization’s numbers can remain intact. suggests that customers submit number porting requests to their incumbent providers one or two weeks before the install date to ensure a smooth transition.

 “We recommend our customers install their SIP trunks with temporary phone numbers to make sure the connection works with their system before their own numbers port in,” said McNicoll. When deploying SIP trunks, the enterprise must also determine its bandwidth needs. Experts on TechTarget’s IT Knowledge Exchange suggest that around 90 Kbps of bandwidth per line of voice traffic, combined with appropriate QoS prioritization, is needed on top of other traffic on the wide area network. However, UC managers should consult with their SIP trunking provider to determine specific requirements.

SIP trunking providers also vary in how they connect to the enterprise's UC platform. SIP trunking utilizes an enterprise’s Internet connection, so the networking team will need to reconfigure the corporate firewall to allow SIP traffic to reach its destination within the enterprise. Simply directing ports to the IP telephony platform, however, could expose the platform to denial of service (DoS) attacks or other security threats. Instead, enterprises should consider deploying session border controllers at the network edge to secure these VoIP links. Enterprises can overcome these challenges to SIP trunking if the UC, WAN and network teams work together to ensure a smooth transition of services.

Interfacing SIP trunks with the IP telephony platform will vary by vendor and the exact enterprise deployment, but most modern UC solutions now provide native SIP support. Connecting legacy circuit-based PBX systems is a different story. An enterprise will need an IP telephony gateway to link the IP-based SIP trunks to the PBX as traditional voice circuits, enabling enterprises on older phone systems to take advantage of the benefits of low cost SIP trunking.

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