Vintage GPO - CHESILVALE Dual Standard Test Set DSTS2

Vintage GPO/Telecom - CHESILVALE Dual Standard Test Set DSTS2. I am a little vague as to the use of this peice equipment as it is from an extensive collection belonging to a friends late husband who worked for GPO/Telecom/BT, I have no idea how to test this to see if it works. Manufacturers information reads:- Low cost test telephone with all the essential features. Selected by network operators and customer equipment installers around the world for its reliability and ease of operation. Robust and durable, the model is equipped with high impact case and belt clip. Features include umlimited line voltage protection, sounder and high impedance monitor. It costs around £100 new. Will be listing other vintage scientific (GPO) items of a similar age over the next few days/weeks.