If you have subscribed to Caller Display, the caller's details will be displayed before you answer. (see below and also the full user guide online at bt.com/ This User Guide provides you with all the instructions on the next few pages. Your BT3520 works by sending radio signals between the handset. Phone manuals and free pdf instructions. Find the user manual you need for your phone and more at ManualsOnline.Documents. BT3520 user guide · BT3520 quick start guide · BT3520 specification sheet · BT3520 CE Declaration of Conformity. We have 1 BT BT3520 manual available for free PDF download: User Manual. BT BT3520 User Manual (50 pages). BT BT3520 Manual Online: Handset Display Icons. These are the symbols you will see on your handset's Handset Display Icons - BT BT3520 User Manual.