Call reminder email notification template

Hello,

Is there an upgrade safe way of adding new custom fields to the email template for call reminders?

I can access the template in include/language/en_us.notify_template.html and customize it by creating custom/include/language/en_us.notify_template.html with the new text and fields. The new text works in the reminder emails, but field values don't (they show as empty). Is there a way to get the field values to work?

Kind regards,

Miska