Hi JIE sorry for the delayed response here. I think in some cases that would work, however I am not sure how we would be able to do that for the instance where a user texts a keyword to opt-in to SMS. An example would be if we put an opt-in message on a direct mail piece with a keyword to opt in, if the user exists in our system, but does not have that phone number in the default phone field then an alias profile is created. This is the case if a user doesnt exist at all - the phone number that opted-in via the keyword will be created as an alias profile. Since this process is automated between Braze and the carriers, I am not sure how we would be able to create an external id prior to an alias being created.