If you have tried Click-To-Dial example, you might wonder if it is possible to call your home phone or cellular phone, besides the SIP phones. It is definitely possible with Voxeo Prism.

In SIP Servlet, a PSTN phone number is represented by a TelURL object based on RFC 3966. For example, here is the code snippet to generate a TelURL object for 407-555-1212.

  URI target = sipFactory.createURI("tel: 1-407-555-1212");

Once you have a TelURL, you can use it the same way as a SipURI, both of which are URI. When Voxeo Prism tries to send a SIP message to a TelURL, it will try to map the telephone number (in E.164 format) to a SIP address based on ENUM.

According to ENUM, a telephone number in E.164 format should be converted to a unique key based the following steps.

1. Remove all characters with the exception of the digits.  For
      example, given the E.164 number " 1-407-555-1212" (which would
      then have been converted into an AUS of " 14075551212"), this
      step would simply remove the leading ' ', producing
      "14075551212".
2. Reverse the order of the digits.  Example: "21215557041"
3. Put dots ('.') between each digit.  Example:
      "2.1.2.1.5.5.5.7.0.4.1"
4. Append the string ".e164.arpa." to the end and interpret as a
      domain name.  Example: 2.1.2.1.5.5.5.7.0.4.1.e164.arpa.

Then the key is used to look up the corresponding SIP address in the DNS NAPTR records. E.g. the following record maps “ 1-407-555-1212” to “sip:14075551212@example.com“.

$ORIGIN 2.1.2.1.5.5.5.7.0.4.1.e164.arpa.
       NAPTR 100 50 "u" "E2U sip"             "!^(\ 14075551212)$!sip:\1@example.com!"    .
       NAPTR 100 51 "u" "E2U h323"            "!^\ 14075551000$!h323:operator@example.com!"    .
       NAPTR 100 52 "u" "E2U email:mailto"    "!^.*$!mailto:info@example.com!"

By default, Voxeo Prism uses the default DNS server configured on the system to perform the ENUM lookup. However, it is possible to specify a specific DNS server for the ENUM lookup. You can even specify a different domain suffix instead of the default “e164.arpa“. Please see Voxeo Prism User Guide for more information on how to configure that.

While ENUM gives you complete control on how to map individual telephone number to different SIP address, another popular way to connect Voxeo Prism to PSTN is through a VoIP (SIP trunking) provider.

Voxeo Prism always looks up ENUM first. If no mapping is found in ENUM and a VoIP provider is configured, the SIP messages will sent to the VoIP provider. The following picture shows where you can configure the VoIP provider’s gateway information.

Obviously, Gateway’s Host, Port, and Protocol specify where the gateway is. If the gateway requires SIP registration, you further configure the Registration section. The registrar’s Host and Port is the same as gateway’s, unless specified differently. The User and Domain allows you to specify the Address-of-Record (AOR) for registration. The Auth User and Password allows you to specify credentials in case the registrar requires authentication.  For more information on specific fields, please refer to Voxeo Prism User Guide.

Originally from Voxeo Blogs