 Between 1988 and 1992, the Digital Equipment team approached more telecommunications equipment manufacturers (including Siemens ROLM and the former AT&T, whose Definity PBX product line is now owned by Avaya) and implemented the additional protocols needed to interoperate with their telephone systems.However, it quickly became clear that the number of proprietary protocols was becoming unmanageable.Call models differed between telephone systems, making it difficult to write CTI-enabled application software.