"Security Requests from Sender" Pop-up notification message ?

Good morning! Has anyone encountered the attached warning / notification before Please? I asked DS Support and they told me it could be either an incorrectly formatted mobile phone number or an invalid mobile phone number. I tested the Incorrectly formatted mobile number but that doesn't / cant be a logical rationale. DS runs default validation relative to your Country Code used...(e.g. In UK, numbers MUST start with a “7” and be 11 digits incl. Dialling code). ANY deviation from these parameters and envelope cant / wont progress. Its difficult to test Invalid number, I tried a few unlikely numbers and envelope was sent, so not really a consideration for this scenario. Any help much appreciated! Tks, Mike