A system comprising: a SIP Proxy Server, and DMR Gateway(s) to interface the SIP Proxy Server with DMR Network(s) matching Private/professional Mobile Radio (PMR) features; each DMR Gateway is univocally assigned with a SIP ID and is designed to: interpret messages from the SIP Proxy Server to manage DMR signalling/data/voice features toward DMR Terminals, and initiate DMR features on account of DMR Terminals operating under it and make requests for DMR signalling/data/voice features toward destinations belonging to another Network; each DMR Gateway is designed to: transcode an over-the-air, manufacturer-specific DMR Terminal registration into a SIP REGISTER message to result in the SIP Proxy server perceiving and managing the DMR Terminal as a SIP User Agent; the SIP Proxy Server is designed to manage: DMR signalling features, including voice call set-up, and DMR data features using the SIP MESSAGE method, and/or DMR signalling/data/voice group features.