Managed SIP Trunks offer the advantage of a closely monitored network maintained to deliver the highest voice quality.
The physical connection to the service provider can vary according to the needs of the enterprise and may range from a single T1 with 1.5Mbps of bandwidth which is sufficient for 23 simultaneous calls, up to and including fiber optic connections with very large capacities. Since voice is very susceptible to delay, often the Managed SIP Trunking Service Provider will deliver service using MPLS (Multi-Protocol Label Switching) which assures that the voice packets will receive delivery precedence over other services being delivered on the same physical connection.
For both of the above reasons, many service providers are willing to write Service Level Agreements (SLAs) that guarantee a certain level of service quality and managed SIP Trunks are often more expensive than those services which are offered over the Public Internet.
The firewall remains critical in this application as a security device between a foreign network and the enterprise network. The firewall provides voice and data security for the enterprise, from other foreign networks. It also monitors and secures the SIP traffic, protecting the IP-PBX from malicious attacks which may range from theft of service to Denial of Service attacks. This is important because any malicious issues on foreign networks can quickly become enterprise issues without a security device in between.
Managed SIP Trunks offer the advantage of a closely monitored network maintained to deliver the highest voice quality. The enterprise needs to ensure that its network is robust and that no internal bottlenecks exist that could reduce voice quality. Enterprises should also consider establishing its own security perimeter to maintain control of its network and who is allowed to use the SIP Trunking services.