A connection device allowing the bidirectional passage of an audio flow and a data flow between two communication systems operating according to different standards: a first TETRA mobile terminal including a PEI (Peripheral Equipment Interface) and a second terminal operating based upon cellular phone standards, comprising: • an integration and interface element, apt to be connected to or to integrate the first TETRA mobile terminal; and • an application substrate decoding an analogic audio flow and a data flow coming from said first TETRA mobile terminal in analogous flows available in the environment of said second terminal, wherein said integration and interface element comprises: • a system for connecting the PEI of the first TETRA terminal to said second terminal through a serial connection, whereby all the AT controls necessary to use the services are made available; • a system for connecting the voice interface of the first TETRA terminal, processing the audio-voice signal produced by the microphone and by the loudspeakers of the first TETRA terminal, to said second terminal through a serial connection, wherein the output audio signal is digitalized and is multiplied by a multiplexer with possible controls or data processed by a microcontroller, and wherein a single inlet data flow is demultiplied (DEMUX) so as to obtain two flows: a data flow and an audio flow; • a system for processing controls for activating services through a firmware implemented in said integration and interface element, translating the request by an application of third parties into the related controls for communicating with the integration and interface element through a multiplexing and, in the opposite route, a demultiplexing; • one or more interfaces of wireless and/or wired digital connection system towards said second cellular device achieving said serial connection. • an integration and interface element, apt to be connected to or to integrate the first TETRA mobile terminal; and • an application substrate decoding an analogic audio flow and a data flow coming from said first TETRA mobile terminal in analogous flows available in the environment of said second terminal, • a system for connecting the PEI of the first TETRA terminal to said second terminal through a serial connection, whereby all the AT controls necessary to use the services are made available; • a system for connecting the voice interface of the first TETRA terminal, processing the audio-voice signal produced by the microphone and by the loudspeakers of the first TETRA terminal, to said second terminal through a serial connection, wherein the output audio signal is digitalized and is multiplied by a multiplexer with possible controls or data processed by a microcontroller, and wherein a single inlet data flow is demultiplied (DEMUX) so as to obtain two flows: a data flow and an audio flow; • a system for processing controls for activating services through a firmware implemented in said integration and interface element, translating the request by an application of third parties into the related controls for communicating with the integration and interface element through a multiplexing and, in the opposite route, a demultiplexing; • one or more interfaces of wireless and/or wired digital connection system towards said second cellular device achieving said serial connection.