# Voice over IP/ENUM and E146 Technology

The following section is quite technical, as it deals with complicated infrastructure and the integration of VoIP into core internet technologies.
The ENUM protocol (described in RFC document RFC 3761) translates standard telephone number routing (known as E.164, as the ITU defines the standard by which telephone numbers are assigned in the E.164 standard) into an addressing scheme able to route to VoIP (eg. SIP/H323) gateways using DNS technology.
ENUM technology allows VoIP servers to use the DNS infrastructure to query routing paths for terminating voice calls given a phone number.
When a call is initiated to a given phone number, a VoIP gateway capable of ENUM service will query the provided number in DNS to determine whether there is a path other than the PSTN line available to the destination service.
This allows providers to advertise paths via VoIP to services otherwise only available via a VoIP to PSTN gateway.
ENUM records may contain other information in addition to a VoIP gateway address, such as a web URL or e-mail address.

### Private vs Public ENUM databases

The public ENUM database is a collection of ENUM records, known as NAPTR RRs in DNS speak, which map telephone numbers to VoIP service gateways. These records are stored in a hierarchical tree beneath the e164.arpa domain.
Records within the e164.arpa domain are delegated to their responsible owner via the E.164 addressing convention. Much like the in-addr.arpa domain, which is used to provide mapping of IP addresses to names, these records are delegated in blocks to the agency responsible for this number assignment.
The agency is then responsible for further smaller delegations where necessary. As a result, it is unlikely that individuals or small businesses will have access to provide ENUM records for their telephone numbers.
It also stands to reason that telephone companies would be hesitant to allow telephone calls destined to their customers to instead terminate over the Internet, as much of a phone company's revenue would be derived from call termination fees, charged to carriers for calls terminating on a company's network.
This is unfortunate for end users. Via the ENUM database, you would have the opportunity to direct VoIP-based calls to terminate over the Internet via your VoIP gateway, rather than via a phone line.
For this reason, several free ENUM databases have been created to allow users to share alternate IP-based paths to their phone numbers. Most ENUM aware gateways will allow additional 'search domains' to be configured, providing access to these unofficial databases.
Whilst configuring a gateway to query these extra domains can dramatically reduce costs by terminating calls over IP, therefore freeing copper lines/voice channels for both the caller and recipient, there is an important factor to consider. If a malicious or unauthenticated entry is added, an attacker can direct calls for a phone number to an arbitrary VoIP gateway.
Consider the example of rival businesses. If Business A were tech-savvy and entered their competitor's number into a public directory, directing calls to their own VoIP gateway, any calls placed via a VoIP gateway configured to query that directory would be directed to Business A's phone, rather than the intended recipient, Business B.
Some of these directories require voice verification of ownership when you register phone numbers. Others do not require verification or have dispute resolution policies which deal with disputes on a case-by-case basis.
Another less serious but still important consideration is the possibility that your information will be harvested from the database, or sold by the database owner. ENUM information may be valuable to advertisers or phone spammers, as it would provide them with a list of valid phone numbers, and allow them to call for free via IP.

### Public ENUM registries
#### VoIP Providers Worldwide

VoIP technology has completely changed how companies interact with their customers. VoIP has scaled so much that now it is being used for residential as well as business purposes. VoIP providers are mainly of two types, Retail VoIP provider and wholesale VoIP provider. There are bunch of VoIP providers online but these are some of the best VoIP providers online.
* Traverse Telecom
* Ringcentral
* Grasshopper

#### ENUM Trial Registries

Many ENUM trial registries are becoming available. Most of these registries are regional, and often are limited by telephony provider, limited number of trial participants, or a trial fee.
Australian ENUM Trial - Fee applies for registration
Austrian ENUM Trial - Open to Austrian residents, a fee applies for registration.
German ENUM Trial
Japanese ENUM Trial - Open to ETJP contributors since 2003. Currently has 46 members.
Taiwan ENUM Trial
UK ENUM Trial
* ↑ Wholesale VoIP Provider