Emergency Services Network

ESN will transmit fast, safe and secure voice, video and data across the 4G network and give first responders immediate access to life-saving data, images and information in live situations and emergencies on the frontline. Investment in ESN will also mean improvements to 4G network coverage, which will enable 999 calls to be made securely from mobile phones in some of the most remote and rural parts of Great Britain.

https://www.gov.uk/government/publications/the-emergency-services-mobile-communications-programme/emergency-services-network