Extensions are at the core of every PBX. Extensions can have multiple numbers, DIDs and ANIs for inbound and outbound call identification. Each extension has a name, a company position and passwords for web and SIP access. Each extension may have its own timezone, web and IVR language and have an image associated with the extension which can be shown on VoIP phones that support showing a photo caller-ID. Extensions can have their own ringtone, permissions for various actions, address book and music on hold source.

The PBX handles basic redirection features like do not disturb (DND) and call forward on busy, on timeout, when there is no registration or of all calls. When calling a specific PBX, other extensions can also be included in the call like in a hunt group for every extension. Users can log in at other extensions, a feature called hot desking. Each extension may have multiple registrations which ring simultaneously when the PBX wants to engage with the user.

Every extension can have multiple mobile phones associated with it. When someone calls the extension, the PBX will immediately or after some time include the mobile as if it was a registered extension. This feature can be limited to specific times during the day, so that users don’t get annoyed by the calls after hours. Mobile phones can also be used to place outbound calls. This makes sure that the extension outbound number is shown on the caller’s display and calls are charged to the PBX, which can be important for international calls. Mobile phones can also be part of hunt and agent groups, further extending the user’s mobility even in environments like call centers or sales and support groups. The PBX can send text messages to the user’s phone, for example when a new voicemail message is available. Those messages can even include the audio recording, if supported by the text provider.

Transferring calls is a core feature of every PBX. The PBX supports various attended and unattended transfers that are triggered by the user’s VoIP phones. In addition to that, calls can be parked on park orbits. In order to avoid caller starvation, the PBX can automatically reassign parked calls or transfer them out of the orbit.

The auto attendant helps to direct incoming callers to the right account on the PBX. It supports a multitude of pre-recorded or custom announcements. Callers can search for users by their name. Together with service flags, the routing can be programmed depending on time of day. The routing can be programmed remotely for days when it is not possible to get to the office (snow days). The auto attendant includes dual language support.

For low call volume groups, hunt groups provide an easy way to distribute calls in a group. Incoming calls are handled in parallel to other incoming calls. The PBX supports three call stages that escalate calls to different group of participants. Ring melodies and music on hold can be assigned specifically for the hunt group. Hunt groups can also include regular phone numbers and cell phones in the stages. Calls can be recorded for each hunt group, and the PBX can send out emails about the calls at the end of the day.

Voicemail is another standard feature that practically every PBX offers. Various announcements, including the name of the extension or a choice of personal greetings can be played. 

When there is a new message, the PBX notifies the user via the message waiting indication of the VoIP phone, by email or by SMS. If a message is not retrieved within a certain time, the PBX can call the user up and deliver the message. This can be escalated to a manager that will be called if the user did not retrieve the message. Messages can be shared with other mailboxes.