Two factor authentication: "number already exist in organization"

Two factor authentication: "number already exist in organization"

Hello,
I am trying to set up two factor authentication for me and my family members. This is all working fine, except that it seems that I cannot use the same phone number as backup number multiple times. All family members have their own mobile phone numbers as primary number, but I want the backup phone number of multiple account to be my phone number, so that they can contact me in case of non-working e-mail.
I understand using the same phone number multiple times is not very secure in a professional organization, but for a family setting this is a silly constraint. Could this constraint be lifted?