In Make create a new Scenario.
Add the Channels' "Watch Incoming Call Status" module.
Click "Add" to create a webhook. Name the webhook and Save. Click "Copy address to clipboard" to copy the webhook URL.
In Channels go to Integrations > Webhook > Add webhooks.
For Event type select "Missed incoming call". Add a name for the webhook. In Webhook URL paste the previously copied URL from Make.
Go back to Make and Add another module. Select the Slack's Create a Message module.
Connect your Slack account with Make, select the channel and message that will be sent. In the text input you can add the data collected from the call like Name, Last name, Phone number or other information so it will appear in the message in Slack.
Save your scenario and make sure it is activated. Now you will receive a new message on Slack every time there's a missed incoming call in Channels.
Note: The Module "Watch Incoming Call Status" will only work with the "Missed incoming call" event type. For other events use the Webhooks' module "Custom webhook".