How can I embed a phone number from a related custom module into an email template.

We have a custom module that contains all the detail information about each one of our stores.  We have created an email template that is sent as a followup to our customers when a purchase is made and we embed their name, email address, etc in the email.  The contact record has a store_id that points to the store record in the custom module and this record has the store phone number in it.  We would like to include this phone number in the body of the email but I do not know how to code this.