feat: add per-inbox toggle to disable incoming calls (#14645)

## Description

Adds a per-inbox "Allow incoming calls" toggle for voice-enabled
WhatsApp and Twilio inboxes. When turned off, the setting is persisted
on the channel; actually rejecting inbound calls is handled in a
follow-up PR.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## Screenshot
<img width="804" height="384" alt="Screenshot 2026-06-04 at 11 44 07 AM"
src="https://github.com/user-attachments/assets/df8bb026-0387-4031-bcba-6d9a56872eb7"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/i18n/locale/en/inboxMgmt.json

@@ -653,6 +653,10 @@
       },
       "CREDENTIALS": {
         "DESCRIPTION": "Voice calling requires Twilio API Key credentials. These are used to generate tokens for agent voice connections."
+      },
+      "INBOUND": {
+        "LABEL": "Allow incoming calls",
+        "DESCRIPTION": "Let customers call this number. When turned off, incoming calls are declined automatically — agents aren't notified and no conversation is created. Agents can still place outgoing calls."
       }
     },
     "WHATSAPP_CALLING": {