Athento (through integration with Viafirma) allows you to digitally sign a document by entering a code that is sent to the signer's phone.
The OTP (One Time Password) signature is based on a one-time use code, generated from the hash of the document to be signed. The verification code is generated with a random algorithm for which the content of the document to be signed (hash) is used, being able to prove that the OTP code corresponds to a specific document and not to another.
An OTP signature is completely legal in the European Union, you can consult information about it on Viafirma's website. https://www.viafirma.com/faq/es/legalidad-firma-otp-sms/