Lync API: Get Caller's Phone Number

When a participant is added to a conversation, I need to be able to get the phone number they used to call in.

It appears I can lookup the caller's Endpoint URI's using Contact.GetContactInformation but this won't tell me what phone number the caller actually called in from.

I forgot to mention that I've been successful in getting it but the Lync API translates the URI to the Contacts DisplayName if the contact is entered as a Contact. Somehow I need a more reliable way to get the callers FROM phone number.