Good morning,
I took a closer look at the API and unless I’m mistaken
With Import contact, it is possible to update " « updateExistingContacts »: true," but it will be done from the email address
To update, emails must match
So you can’t update a contact with https://api.brevo.com/v3/contacts/import (https://developers.brevo.com/reference/importcontacts-1)from the ext_id or sms
Personally I use https://api.brevo.com/v3/contacts (https://developers.brevo.com/reference/createcontact) with the update activated with « updateEnabled »: true
Values are email or sms or ext_id
I hope this little note was useful to you 
Have a good day
Philippe
Note: be careful to put all your numbers in integer format and not string. Brevo recognizes that it makes a difference. For this I use the php function FILTER_VALIDATE_INT