ENUM links a telephone number to a host or resources on the Internet that can connect the call, either end-to-end over IP networks or through a designated gateway to the PSTN. This is useful for connecting SIP- or H.323-compatible endpoints that exist across domain boundaries.

The potential applications of ENUM are far-reaching; however, the principal applications for ENUM have centered on two areas.
One is Voice over IP (VoIP) and the other centers on Voice Protocol for Internet Mail (VPIM). The long-stated goal of the VoIP industry has been to make a phone call over the Internet as easy to make and as high-quality as a regular PSTN phone call.
The goal of the VPIM industry has been to develop a comprehensive mechanism by which voice mail systems could exchange messages over IP networks. ENUM enables carrier and enterprise voice mail systems to find each other, interoperate, and exchange messages.

Although VoIP and VPIM are the most widely discussed applications, other application possibilities for ENUM, including but not limited to Internet Fax and Instant Messaging, exist.