feat(voice): reuse open conversation for Twilio outbound calls

Accept an optional conversation_id in POST /contacts/:id/call. When the
hint matches the picked voice inbox, OutboundCallBuilder reuses that
conversation instead of creating a new one — mirroring the WhatsApp
calling flow so the agent stays in the same thread.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

enterprise/app/controllers/api/v1/accounts/contacts/calls_controller.rb

@@ -10,7 +10,8 @@ class Api::V1::Accounts::Contacts::CallsController < Api::V1::Accounts::BaseCont
       account: Current.account,
       inbox: voice_inbox,
       user: Current.user,
-      contact: contact
+      contact: contact,
+      conversation: existing_conversation
     )
 
     render json: {
@@ -38,4 +39,13 @@ class Api::V1::Accounts::Contacts::CallsController < Api::V1::Accounts::BaseCont
       inbox
     end
   end
+
+  # Reuse the open conversation when the caller is already inside it; ignore
+  # the hint if it doesn't belong to the picked voice inbox.
+  def existing_conversation
+    return nil if params[:conversation_id].blank?
+
+    conversation = Current.account.conversations.find_by(display_id: params[:conversation_id])
+    conversation if conversation && conversation.inbox_id == voice_inbox.id
+  end
 end

enterprise/app/services/voice/outbound_call_builder.rb

@@ -1,15 +1,16 @@
 class Voice::OutboundCallBuilder
   attr_reader :account, :inbox, :user, :contact
 
-  def self.perform!(account:, inbox:, user:, contact:)
-    new(account: account, inbox: inbox, user: user, contact: contact).perform!
+  def self.perform!(account:, inbox:, user:, contact:, conversation: nil)
+    new(account: account, inbox: inbox, user: user, contact: contact, conversation: conversation).perform!
   end
 
-  def initialize(account:, inbox:, user:, contact:)
+  def initialize(account:, inbox:, user:, contact:, conversation: nil)
     @account = account
     @inbox = inbox
     @user = user
     @contact = contact
+    @existing_conversation = conversation
   end
 
   def perform!
@@ -18,7 +19,7 @@ class Voice::OutboundCallBuilder
 
     ActiveRecord::Base.transaction do
       contact_inbox = ensure_contact_inbox!
-      conversation = create_conversation!(contact_inbox)
+      conversation = @existing_conversation || create_conversation!(contact_inbox)
       call_sid = initiate_call!
       call = create_call!(conversation, call_sid)
       message = Voice::CallMessageBuilder.new(call).perform!