Not able to update phone number

Hi, My HMRC account is linked to an old phone number which is not active anymore. I have tried to reset the number online as well as using the HMRC chat agent and the phone support, however as I am not able to answer one of the security questions I am not able to pass the security check. The security question that I am not able to answer is which month and year I initially registered for self-assessment and unfortunately I am not able to retrieve this information from my documents. I am struggling to get any information on who I could contact now in order to reset my security details, for example whether I can escalate my problem somewhere or whether I need to send a letter to someone within HMRC. Any advise and help would be highly appreciated. Thanks!

Hi TB1983,
If you are unable to self serve online to reset your phone number you will need to contact our Online Heldesk for assistance, security will need
to be completed before the number can be reset.
Technical support with HMRC online services. 
Thank you,.

Hi, Thank you for your answer. However the whole point is that I am not able to pass the security questions and hence HMRC online help is not able to help me to reset my phone number. Additionally no one from HMRC has been willing to direct me to any further help how I could actually reset my login details (maybe by sending a letter etc.). So my question is who to reach out to when NOT able to pass the security questions online/on the phone? Thanks!

Hi
If you are unable to self serve to reset your telephone number then you would need to contact our online helpdesk for further assistance.
You can contact HMRC atTechnical support with HMRC online services