It seems that a user can make phone calls or send text messages even when in a locked state by asking Siri to make phone calls or send text messages.
Saying things like “Call this number”, “Read my unread emails”, “Read my last email” or “Read my last text message” lead to Siri insisting that you unlock your phone.
Siri’s behavior can become a privacy issue, because anybody can send a text message or an email message using your phone while your phone is locked. Here is how:
1-Active Siri while your phone is locked
2-Say “send a text message”
3-Siri will reply: to whom should I send your message?
4-Say a contact name: Stacey
5-Siri will respond: what do you want to say?
6-Say your message
7-Just confirm and Siri will send your message.
This is not limited to sending text messages. Siri can also send emails as well while your phone is locked by telling Siri “Send an email”.
This means that anybody can a mail or text using your phone without knowledge of the passcode and/or Face ID or Touch ID.
You can disable Siri access on the lock screen by going to Settings > Siri & Search and toggle off “Allow Siri When Locked”.
See also: Siri not working?