You can add a user property: https://confluence.atlassian.com/display/JIRA/Managing+Users#ManagingUsers-Addingapropertytoauser
As mentioned in the docs, it doesn't serve any functionality but you can find the phone number in he profile once it is added.
Hey admins! I’m Dave, Principal Product Manager here at Atlassian working on our cloud platform and security products. Cloud security is a moving target. As you adopt more products, employees consta...
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