I need to add mobile number custom fields to my user profiles if that is possible without additional addon,
Hello,
You can use user properties. You can find more info here:
https://confluence.atlassian.com/crowd/specifying-a-user-s-attributes-18579612.html
But you will not be able to see this attribute in the issue create, edit or view screens.
If you want to show this info in an issue screen, you should use an add-on. You can try the Power Custom Fields add-on. It is a free add-on:
https://marketplace.atlassian.com/apps/1210749/power-custom-fields?hosting=server&tab=overview
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