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SIP Trunks Find a Niche: Page 3 of 5

The Sip Standard Dilemma

Conventional telco service delivery standards are well-established. For POTS lines, the protocol is very simple, and compliance is not difficult. Higher-capacity solutions such as T1 PRIs, though technically more complicated, have been around long enough that the communication protocols are observed universally.

Not so for SIP trunks. SIP as a protocol works great, but proper functionality is contingent on compatible implementation on both client and provider sides. To address potential problems, many SIP VSPs provide customers with preapproved equipment or a list of compatible equipment to select from.

Because we didn't fully understand the compatibility concerns, we took the reverse approach: We purchased hardware based on functional requirements, then looked for a compatible VSP. Learn from our mistake: Many service providers claimed they would support our Cisco Call Manager Express IP PBX, but upon establishing service, many of the advanced call features failed to work. Among the five SIP VSPs we tried, only one was able to properly accommodate our IP PBX.

There is one major shortcoming with Cisco's SIP trunking implementation: As of this writing, it can handle SIP transfers and forwards only through a proxy--in this case, a VSP. All transfers and forwards using Cisco's hardware require a customized setup for each customer.