What type of contacts are possible to check?

In iMessage and FaceTime, a contact can be either a phone number or an email address. Most often, this mailbox is the user’s Apple ID. But users can also add other emails as contact info for conversations or calls.
You can read more about this in this article from Apple:
https://support.apple.com/101744

In our service, you can submit lookup both by lists of emails and by phone numbers. If this email is activated for a contact in iMessage or FaceTime, you will get positive results.