DNS-based ENUM can be used to set up connections between endpoints in telephone infrastructure. ENUM in VoIP uses DNS queries to determine which servers should be contacted for setting up a VoIP session for a given telephone number.
In many countries, operators are obliged to provide number portability. Number portability lets subscribers keep their number when switching operators (IETF rfc 4694). ENUM is used to provide the central lookup systems for such number portability, based on DNS.
Use in Mobile networks
ENUM enables voice calls as part of VoIP implementations in modern mobile 4G and 5G networks (VoLTE). DNS supports ENUM to resolve and connect VoIP calls, as further explained by the GSMA ENUM Guidelines.