hi mchv2.0,
my understanding is that you keep seeing this prompt when log in to http://portal.office.com , if there is anything misunderstood, please correct me.

to narrow down the issue, please help to confirm the following information:
- are the users accounts in your tenant synced from on premise active directory or created from office 365 admin center > active users?
if it is the former scenario, please add the mobile phone number at the local ad and then sync to office 365. you can refer to jason’s reply
here.
- does the prompt only pop up to admin account(s) or all user accounts?
if the prompt only shows up to admins, please ask them to change the alternate email address to a new one under
office 365 settings > personal info and check the outcome.
3. have the affected users finished configuring the mobile number and phone number under
office 365 settings > personal info ?
- if it is not the above symptom, please capture a screenshot of it. to upload a screenshot, you can click
use rich formatting under the reply box in the forum and then click the icon of insert/edit media
> browse > insert.
regards,
james