Many organisations, particularly the larger ones, have telephony infrastructures that have evolved over time, often as a result of mergers and acquisitions. Typically, they will have a number of different PBXs from different manufacturers and, quite possibly, a mix of traditional TDM and IP technology.

Different user extensions will support a different set of features, depending on the handset model and on which PBX it is connected. Staff working in a different part of the site to normal may find an unfamiliar user experience when trying to use a phone. They may even be unable to access their own corporate voice mailbox.

In the city of London, a survey by TeleWare in 2008 showed that 80% of large corporates had 2 or more disparate PBX systems, requiring separate maintenance and support as well as providing an inconsistent delivery of services across the organisation.