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

spec/models/channel/whatsapp_spec.rb

@@ -248,4 +248,22 @@ RSpec.describe Channel::Whatsapp do
       expect(channel.voice_enabled?).to be false
     end
   end
+
+  describe '#inbound_calls_enabled?' do
+    let(:account) { create(:account) }
+
+    it 'returns true by default when nothing has been toggled' do
+      channel = create(:channel_whatsapp, account: account, provider: 'whatsapp_cloud',
+                                          validate_provider_config: false, sync_templates: false)
+      expect(channel.inbound_calls_enabled?).to be true
+    end
+
+    it 'returns false only when explicitly disabled in provider_config' do
+      channel = create(:channel_whatsapp, account: account, provider: 'whatsapp_cloud',
+                                          validate_provider_config: false, sync_templates: false)
+      channel.update!(provider_config: channel.provider_config.merge('inbound_calls_enabled' => false))
+
+      expect(channel.inbound_calls_enabled?).to be false
+    end
+  end
 end