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/views/api/v1/models/_inbox.json.jbuilder

@@ -140,6 +140,7 @@ end
 ## Voice attributes for TwilioSms
 if resource.twilio? && resource.channel.respond_to?(:voice_enabled?)
   json.voice_enabled resource.channel.voice_enabled?
+  json.inbound_calls_enabled resource.channel.inbound_calls_enabled?
   json.voice_configured resource.channel.try(:twiml_app_sid).present?
   json.has_api_key_secret resource.channel.try(:api_key_secret).present?
   if resource.channel.try(:twiml_app_sid).present?
@@ -149,4 +150,7 @@ if resource.twilio? && resource.channel.respond_to?(:voice_enabled?)
 end
 
 ## Voice attribute for WhatsApp Cloud (only embedded-signup channels surface true)
-json.voice_enabled resource.channel.voice_enabled? if resource.channel_type == 'Channel::Whatsapp' && resource.channel.respond_to?(:voice_enabled?)
+if resource.channel_type == 'Channel::Whatsapp' && resource.channel.respond_to?(:voice_enabled?)
+  json.voice_enabled resource.channel.voice_enabled?
+  json.inbound_calls_enabled resource.channel.inbound_calls_enabled?
+end