BLF stands for Busy Lamp Field. It is a feature in a phone system that allows users to see the status of other extensions on the same system. This can be useful for determining whether someone is available to take a call or if they are already on the phone.

The BLF feature is typically displayed on a phone's screen as a list of extensions, with each extension showing a status such as "available," or "on a call," Users can then see at a glance which extensions are available to take a call and which ones are busy.

Connexium's phone system allows users to set up custom BLF buttons on their phone, which can be used to check the status of a specific extension or to initiate a call to that extension or any other external number (you cannot see the status of an external number). This can be particularly useful in busy environments where it is important to be able to quickly connect with other team members.