The Telecom Industry Association (TIA) reckons it has proved the interoperability of IP PBXs and legacy TDM PBXs. It says users can be sure of interworking via the DPNSS (Digital Private Network Signalling Systems) protocol, meaning that the IP kit can be safely mixed with older gear.

"We have demonstrated that there is a good migration path now from TDM to IP - it doesn't have to be 'rip and replace'," said Alan Cobb, the TIA's director general, presenting the test report. He added that the need for migration is shown by the market's preference for hybrid IP-capable PBXs which will outsell IP-only PBXs by four to one this year.

Four companies took part in the interoperability testing - Alcatel, Mitel and Siemens all set up IP PBXs, while Westell supplied its InterChange gateway device partnered with a Cisco CallManager. Between them, they paid around £100,000 for BT Exact do the test work at its Martlesham labs, testing them against TDM PBXs from Ericsson, Mitel, Nortel and Siemens.

There are caveats, though. First off, five percent of the interoperability tests failed, according to Richard Swale, the consulting engineer from BT Exact in charge of the test work. He said some of the failures were because the PBXs were tested as delivered, so they could have been reconfigured to work, while others were down to 'supplementary features' not supported by TDM PBXs.

"In terms of basic call handling, that worked out of the box," he said. "The issues come in corner cases - the vast majority can be configured to work. The key is what could be done with the system, compared with the things supported in DPNSS, for example some features are not supported at the interworking point."

The other issue is that only three IP PBX vendors took part, plus they were mostly those with a TDM heritage and an interest in pushing migration rather than rip and replace. Cobb admitted that the TIA would have liked more vendors involved - Avaya, Cisco and Nortel were all absent despite taking part in an earlier round of IP-only compatibility testing.

He could not say why they hadn't joined in this time, but said he expected a much bigger pool of suppliers to take part in the next phase of testing, when the TIA plans to look at SIP as the next-generation signalling protocol.

This phase 3 of testing is going to be much bigger, as it will mean testing handsets and maybe even softphones on PCs and PDAs for interoperability, as well as PBXs.

All in all, the test results are good news for IP telephony migration. "We have shown that you can do multivendor, with caveats," said Richard Swale. "If you're doing complex things you really do need to sit down and plan, just as with a TDM PBX you would have to set up numbering plans and so on."

You also need to test IP telephony on a live network, he warned: "The tests assumed a perfectly clear IP network, but real networks aren't truly transparent end to end, because of firewalls and so on."