Hi @Theo Gordon ,
Are you referring to phone number assigned in Teams?
As far as I know, it's not feasible to retrieve the data from a "deleted" user. If it's a recently deleted user(within 30 days), you can try restoring the user's account and then connect to Teams powershell, run the command below to get its phone number:
Get-CsOnlineUser <UserIdentity> | ft LineURI
But if the user has already been deleted permanently, I am afraid it cannot be retrieved any more.
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