Comment by wolvoleo

Yes I know but when you want to link a WhatsApp Web client to a phone with WhatsApp, SMS is not used. You need to scan a QR with the phone in question and sign in to the phone.

SMS is used to register WhatsApp to a new phone, but that signs the original phone out. Also, you need the pin code that WhatsApp forces you to set. If you don't have it you have to wait a week if you got a recycled number. Also the original account holder is notified you tried it.

So this method cannot be used by the police to snoop unnoticed. I believe signal works the same as WhatsApp here, it also forces you to set a pin now.

For telegram this can be used yes but you can set an optional extra password. And also like I said telegram is not E2EE anyway so it's open to warrants.