ENUM (E.164 Number Mapping as specified RFC 6116, https://tools.ietf.org/rfc/rfc6116.txt) is a system that uses the Domain Name Service (DNS) to translate telephone numbers, like '+12025552600', into URIs. A TelURL or SipURI with a user=phone parameter constitutes a telephone number. Often, SIP servlet applications need to resolve such telephone numbers using ENUM as specified in RFC 3824, https://tools.ietf.org/html/rfc3284. The SIP servlet API provides a DnsResolver interface to resolve telephone numbers in a TelURL or SipURI that has a user=phone parameter. DnsResolver is implemented by Converged Application Server, and is made available to applications as a ServletContext parameter with the name javax.servlet.sip.DnsResolver. It can also be accessed using resource injection as described in "Annotation for DnsResolver Injection". After a SIP servlet obtains a DnsResolver, it can use the following methods to resolve the telephone numbers to a SipURI:

• SipURI resolveToSipURI(URI uri)
• List<SipURI> resolveToSipURIs(URI uri)
• List<String> resolveToStrings(URI uri, String enumService)

DnsResolver also contains utility methods that help applications while resolving the URIs. The toEnum(URI uri) method helps applications to get the representation of a URI in ENUM format. The resolvesInternally (SipURI uri) method helps applications decide whether the SipURI resolves to the internal container.