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The two most common explanations vendors gave for features not working were that the third-party
phone does not support that feature or that the SIP standard does not support that feature. There is some truth to these statements. For example, IP PBX vendors had trouble delivering distinctive ring tones to the third-party phones under test. Ring tones are generated locally by the SIP device, so they depend on how the phone manufacturer implements them (for a list of third-party phones used, see "IP Phone Choices," right).

As for SIP, it's young and still growing (see "VoIP: Join the Party" for more on SIP's progress). For example, the SIP standard doesn't detail a signal when a phone goes off hook, so we wanted to see how vendors would handle that. In tests, we placed a call over the Avaya system to a Cisco IP phone we had taken off hook. The phone rang instead of generating a busy signal and forwarding the call to voicemail. However, the same test with the Grandstream and SNOM phones on the Avaya system sent the call to voicemail, which is what we'd expected. An extension of RFC 3265 will address this problem, and Avaya told us it will accommodate the feature in its next release. The other vendors did not get back to us. But note that the presence capabilities of soft phones in this review could identify when their "bound" phone was off hook and generate a SIP "notify" signal for a "busy" status.

IP Phone Choices

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None of the systems received a perfect score in our interoperability tests (see "How We Tested SIP-Compliant IP PBXs,"). Although Avaya's came out on top, it had difficulties with managed-call and blind-call transfers on the Cisco phone. Zultys' setup suffered from its choice of Microsoft Messenger as an endpoint and for bring- ing only two third-party SIP phones. Vonexus also didn't bring the required phones to test, and it got off to a false start with an installation problem stemming from insufficient HMP (Host Media Processing) resources to satisfy our test scenarios; in addition, it didn't include the add-on conferencing module specified in our RFP. Finally, Vonexus was the only vendor to hazard testing using an analog phone, an Audiocode eight-port FXS gateway.

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