The directory within the BT One Phone portal and on the BT One Phone mobile app gives you contact information for all your colleagues, including their BT One Phone presence status and whether they're available to take a call.
In the BT One Phone portal, click on Call Settings and then Directory in the menu bar. To expand the directory structure, just click on the ‘+’ symbol to the left of your company's name.
In the mobile app just select Directory in the main menu. In Settings you can turn on Local directory sync which downloads your contacts from the company directory and stores them in your phone's local address book.