Hi @Veli-Jussi Raitila , not all strings are customizable unfortunately. The strings that are not customizable are part of the built-in user flows and are not exposed for customization. The error message you mentioned, "The value entered needs to be a phone number," is one of those strings that cannot be customized.
"success_verify_code_msg" is a valid ID so it should work. Make sure your page layout version is >= 2.1.0 and that you meet the VerificationControl requirements.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James