PABX suppliers offering the DPNSS digital private network switching system advanced switching capability have bowed to user pressure and will make available a conformance testing suite for products. Users have complained that most of the advanced features promised by the technology such as call-back when free, camp-on and call conferencing, are not available where users of PABXs from different suppliers try to link their exchanges together and use them. The available facilities of DPNSS have therefore been limited to basic call set up between different switch types, despite manufacturers claims to have achieved DPNSS inter-working. But the DPNSS Working Committee, which brings together nine PABX manufacturers, has now approved the development of a specification for an impartial conformance test designed to ensure interworking of their exchanges at a high level. All pabxs claiming to to offer DPNSS interworking will have to conform. The test suite will be publicly available but will be under the control of a vendor-led committee. Independent conformance testing bodies such as the Networking Centre are unlikely to volunteer because of the expense involved in the scheme said a source, who also claimed that vendors are now genuinely interested in inter-working their products. In the past user complaints have gone unheard because DPNSS is a private networking facility and lies outside the jurisdiction of the UK state Office of Telecommunications, Oftel watchdog A member of the working group said he expected that suppliers would have to modify existing products to comply with the new test.