Where is this contact list and how to change it?

I inherited from a previous employee a contact list of customers so that when we need to send an email to a customer we can type in that customers number and the "To" field will populate with their company name or email address.  For instance, we will see either  1424 - AnyCompany or 1424 - Mary Smith (*** Email address is removed for privacy ***).  However, when I go to my address book I can not find this contact list.  So when a customer wants to change the name of the contact, I have to keep a list on paper because I've tried clicking on the contact name in the email and changing it, but then I lose the ability to populate the "To" field by the customer number going forward.  Also, if the customer responds to me after I've sent the email, I lose the ability to send future emails using the customer number. As though that has been overridden. I've also tried adding new contacts by making business cards, but when I go to send an email to them and I start to type in their customer number, nothing comes up.  Since we have over 150 customers and some have 2 or 3 contacts, I can not remember everyone's name.

Unfortunately, we do not know where the contact list is "housed" and how to go about adding or changing it in Outlook. Its not part of the Global Address Book or  the Outlook Address Book.  If you can shed some light on where it is and how to change it, that would be appreciated as it will make my job much easier.

Thank you,

Carolyn