There are, in the business world, a huge number of legacy telephone systems that use a “key and lamp” principle of operation – so-called key telephone systems. Many small business phone systems available in the market today still use this principle of operation. They are extremely ubiquitous and a large number of users and suppliers are very familiar with their principle working concept – Shared Line Appearances.
Existing users will often be quite resistant to losing this feature – they understand it and feel comfortable with it. Take it away and, even if you give them a whole lot of extra new clever features, they will be unhappy. Deals are lost (possibly in large numbers) on the strength of this one feature not being present.
Visual representation of call activity
In fairness, it should be pointed out that SLA provides a very nice visual interface for users. The lamps on each phone provide an immediate indication of current call activity and, given enough programmable keys on the IP phones, an Asterisk based system would be able to show the status of every trunk line and every extension on your system.
Highly configurable ring groups
Asterisk SLA offers a rich combination of options for delivery of incoming calls. It is possible to include or exclude each “station” (extension) in the ring group for every trunk line. Furthermore, you can assign different ring delays and ring durations per station and per line. In many ways it offers a superior set of features for configuring ring groups/pickup groups than the native features of Asterisk.