The 'Phones' you are referring to are 'Audio Logs' in the ODST Mombasa streets section of the campaign. These are accounts of a woman called 'Sadie' who the logs follow throughout the Covenant invasion, I will not spoil it for you but they
come in many forms - whether it is a payphone, ATM machine or another large screen.
They are highlighted with a yellow outline in VISR mode, to access them you need to get close to them and
hold RB or whatever button you use to pick up weapons.