The Authority Enterprise OFFICIAL voice system is ageing and requires modernisation. The future end state is a hybrid solution that fuses on-premises calling capability with the flexibility of a cloud-native call management service. This procurement is for the design, build, migration, and operation of the end state architecture whilst ensuring the users maintain capability throughout the process. This encompasses design and delivery of a cloud native call management core (Software as a Solution (SaaS) or UK zoned public cloud), migration of existing gateways and SIP native Private Automatic Branch Exchange (PABX), modernisation and Session Initiation Protocol (SIP) enablement or replacement of legacy, non SIP PABX, through life support for legacy PABX estate until migration, resilient public telephony breakout and partner connectivity, local survivability for specific sites, a field engineering competence to ensure compliance with Authority processes and service management. The existing PABX estate is mostly analogue lines with a small amount of digital and Voice Over Internet Protocol (VoIP) ; it is not envisaged that this will change due to the cost and complexity of replacing the existing cabling. Planning assumptions: ~113 sites; ~180 nodes; . up to ~65,000 telephone endpoint is any physical or virtual device that originates or receives calls. Anticipated contract term: 5 years. Suppliers should treat volumes and dates as planning assumptions only. Classification: Official Type: Services (telecommunications / IT services). Volume: Planning assumptions above. How supplied: Design, build, migrate, operate; hybrid delivery model (SaaS Voice Platform + on prem survivability where required); Lotting model; Lot A overhaul - design, build & operate. Lot B Legacy support.