A method of exchanging data between first and second telephony terminals during a telephony call, comprising the steps of: a. initiating said call between said terminals using the Session Initiation Protocol, SIP; b. said first terminal generating information to be exchanged; and c. said information being transmitted from said first terminal to said second terminal as a message using SIP; characterised by: generating said information in a format in which each item of information is defined in terms of resources referenced using uniform resource indicators, URIs; wherein said first telephony terminal is a telephony interface of an automated telephony application and said second telephony terminal is a telephony interface of a user in communication with said automated telephony application; wherein said information to be exchanged describes a process of said automated telephony application, comprising a model of a menu structure available to a user of said second telephony terminal, the method further comprising the steps, carried out by said second terminal, of: d. providing a representation of said model to said user of said second terminal; e. receiving from said user an input indicating an interaction with said model; f. generating from said input an instruction to said first terminal, wherein said instruction is generated in a format in which said input is defined in terms of resources referenced using uniform resource indicators, URIs; and g. transmitting said instruction from said second terminal to said first terminal as a message using SIP; whereby said user of said second terminal can interact with and control said telephony application according to said instruction. a. initiating said call between said terminals using the Session Initiation Protocol, SIP; b. said first terminal generating information to be exchanged; and c. said information being transmitted from said first terminal to said second terminal as a message using SIP; generating said information in a format in which each item of information is defined in terms of resources referenced using uniform resource indicators, URIs; wherein said first telephony terminal is a telephony interface of an automated telephony application and said second telephony terminal is a telephony interface of a user in communication with said automated telephony application; wherein said information to be exchanged describes a process of said automated telephony application, comprising a model of a menu structure available to a user of said second telephony terminal, the method further comprising the steps, carried out by said second terminal, of: d. providing a representation of said model to said user of said second terminal; e. receiving from said user an input indicating an interaction with said model; f. generating from said input an instruction to said first terminal, wherein said instruction is generated in a format in which said input is defined in terms of resources referenced using uniform resource indicators, URIs; and g. transmitting said instruction from said second terminal to said first terminal as a message using SIP; d. providing a representation of said model to said user of said second terminal; e. receiving from said user an input indicating an interaction with said model; f. generating from said input an instruction to said first terminal, wherein said instruction is generated in a format in which said input is defined in terms of resources referenced using uniform resource indicators, URIs; and g. transmitting said instruction from said second terminal to said first terminal as a message using SIP; whereby said user of said second terminal can interact with and control said telephony application according to said instruction.