If the phone was issued by the company they own it and all of the content on it. You should not put anything personal on it unless they explicitly have a policy that allows personal content. Most people with company phones have a separate personal phone.
To try to answer your question, any contacts associated with your company email account are visible to the administrator, including any personal contacts that you save to it. They can see all emails sent using the company account. They generally can't see your photos unless you email them using a company email account. Or your texts. They can see your calendar entries for calendars tied to your Exchange account. From there it's mostly common sense. They can see notes associated with Exchange, but not notes that are local on your phone or that sync to your personal iCloud account. However, if you use an iCloud account provided by the company they can see all content that you sync to iCloud.
So the best advice is in my first paragraph, and SravanKrA's post.