For secure use of yahoo account i suggest you to prompt yahoo user to provide a proof that he posses the physical sim card associated with t
For secure use of yahoo account I suggest you to prompt yahoo user to provide a proof that he posses the physical sim card associated with that msisdn he declared as a recovery phone number at the moment of creating a new yahoo account.
In some cases mobile service provider move that msisdn to a new customer by providing that new customer with providing him with a new sim card associated with new ICCID code but associated with the same mobile number, in that case the first Yahoo user can not access his account but the second user account can access the account of the first user because he has the mobile number which was in hands of the first yahoo user.
To avoid this problem yahoo has obligations to prompt the new yahoo user to provide a mobile number (MSISDN) and the proof for having that MSISDN I mean the ICCID code printed on the SIM card and a photo of that code. Even if the mobile service provider moved that MSISDN to another client, they will, of course , provide him with a new physical card havind a different ICCID code. in that case, the later yahoo user will not be able to access the former yahoo user because yahoo will detect that he will provide a differnt ICCID code when he will be prompted to provide the iccid code.
In few words Yahoo will prevent the missusing of the same mobile number by two different persons using the same mobile number

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If you're having account or sign-in issues, please contact our Customer Care team at help.yahoo.com. They are separate from the Yahoo Mail team. They resolve these types of issues and have the tools to help. Hope this helps. Thanks!