It is a common practice that user attributes like phone number, departments etc are synchronized from Active Directory into User Properties within Jira.
To add some personal touch to your email signatures, you may want to include not only the email address, but these extra attributes as well.
Luckily you can access the user properties in your email templates. References to user properties by property name must be prefixed with "jira.meta." as the Jira API requires a fully qualified name of the property.
E.g. If the user has a property, called "phone", the phone property of the current user may be rendered as
Example
$!userPropertyManager.getPropertySet($!currentUser).getString("jira.meta.phone")
Instead of $!currentUser you can use any user objects from the issue, like: $!issue.assignee or $!issue.reporter.