help: Document voice calls option.

Documents the new audio call option (video call with video muted by
default) to the compose box.

Fixes #26895.

help/include/sidebar_index.md

@@ -77,7 +77,7 @@
 * [Polls](/help/create-a-poll)
 * [Collaborative to-do lists](/help/collaborative-to-do-lists)
 * [Global times](/help/global-times)
-* [Video calls](/help/start-a-call)
+* [Start a call](/help/start-a-call)
 
 ## Sending messages
 * [Open the compose box](/help/open-the-compose-box)

help/start-a-call.md

@@ -1,4 +1,6 @@
-# Start a video call
+# Start a call
+
+## Start a video call
 
 {start_tabs}
 
@@ -42,24 +44,51 @@
 
 {end_tabs}
 
-## Change your video call provider
+## Start a voice call
+
+{start_tabs}
+
+{tab|desktop-web}
+
+{!start-composing.md!}
+
+1. Click the **phone** (<i class="fa fa-phone"></i>) icon at the
+   bottom of the compose box. This will insert a **Join voice call.** link into
+   your message.
+
+1. Send the message.
+
+1. Click on the link in the message to start or join the call.
+
+!!! tip ""
+
+    You can replace the "Join voice call." label for the link with any text you
+    like.
+
+{end_tabs}
+
+## Change your call provider
 
 By default, Zulip integrates with
 [Jitsi Meet](https://jitsi.org/jitsi-meet/), a fully-encrypted, 100% open
 source video conferencing solution. Organization administrators can also
-change the organization's video call provider. The video call providers
-supported by zulip are:
+change the organization's call provider. The call providers
+supported by Zulip are:
 
 * [Jitsi Meet](/integrations/doc/jitsi)
 * [Zoom integration](/integrations/doc/zoom)
 * [BigBlueButton integration](/integrations/doc/big-blue-button)
 
+If you choose BigBlueButton as the call provider, there will be a single button
+(<i class="fa fa-video-camera"></i>) for starting a call. The call is initiated
+with cameras turned off.
+
 !!! tip ""
 
-    It is also possible to disable the video call button for your organization
+    It is also possible to disable the video and voice call buttons for your organization
     by setting the provider to "None".
 
-### Change your organization's video call provider
+### Change your organization's call provider
 
 {start_tabs}
 
@@ -75,7 +104,7 @@ supported by zulip are:
 ### Use a self-hosted instance of Jitsi Meet
 
 Zulip uses the [cloud version of Jitsi Meet](https://meet.jit.si/)
-as its default video call provider. You can also use a self-hosted
+as its default call provider. You can also use a self-hosted
 instance of Jitsi Meet.
 
 {start_tabs}