fix(voice): require conversation to belong to dialed contact

Reject conversation_id hints that point to a different contact in the
same voice inbox, so reuse only happens when the agent is genuinely
inside the open thread for the dialed contact.

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

@@ -41,11 +41,14 @@ class Api::V1::Accounts::Contacts::CallsController < Api::V1::Accounts::BaseCont
   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.
+  # the hint if it doesn't belong to the picked voice inbox or dialed contact.
   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
+    return nil unless conversation
+    return nil unless conversation.inbox_id == voice_inbox.id && conversation.contact_id == contact.id
+
+    conversation
   end
 end