Glossary
What is OTP (One-time password)?
OTPOTP is a short code sent to a phone to confirm that whoever is using a service has that phone. It proves possession of the phone at that moment, not that a person agreed to a particular document.
OTPs are good for sign-in. As proof of agreement they are weak, because the code is not tied to the document, and anyone holding the phone can enter it.
A digital signature is tied to the exact file and to a named certificate holder.