How do I change the phone number associated with my Gateway ID (or add a 2nd phone number)

When I login with my Gateway ID it calls me on my home phone with a 6-digit code which I then enter online to verify the login. Once logged in I can change the email address and password associated with the Gateway ID. But how do I change the phone number (or add a mobile number) so that I can a login with my Gateway ID whether I am at home or away from home. It doesn't seem to offer that as an option, or am I missing something?

Hi 

If you are unable to self serve online to change your number you will need to contact my colleagues on the online helpdesk Technical support with HMRC online services

Thank you