Nu Echo Blog

I’m a big fan of both Asterisk and VoiceXML. Each has its own sweet spot. Asterisk is great for building complete telephony systems (dial plans, conference calls, queues, voicemail, etc.), while VoiceXML is the standard way to develop full-blown telephony applications for large organisations.

But what if you want to bridge the two? There are situations where that would make sense. Consider a company using Asterisk as their front PBX. Now if they want to add a speech-enabled auto-attendant or some other self-service application, they could use a VoiceXML platform to run it instead of coding it in the Asterisk dialplan language. Of course, one could do the same using the Asterisk Gateway Interface (AGI) protocol, but he would be limited to the capabilities of the Asterisk dialplan language. (For instance, the generic speech recognizer API only returns the matched text of each NBest, not the semantic interpretation. This can be ok for some trivial applications, but that’s clearly inadequate for serious speech application development.)

The other day, I decided to test this idea and try using a VoiceXML gateway (Voxeo Prophecy in this case) from behind Asterisk. Here is how I made things work.

Machine setup

My setup consists of a laptop running Ubuntu 9.04 with Asterisk 1.4.21. Since Prophecy is only supported on CentOS and RedHat Enterprise Edition, I decided to run Prophecy on CentOS 5.5 inside a VMware virtual machine. The guest machine is configured to use a dedicated network between the guest and the host (the Host-only network configuration):

VMware guest network configuration

Asterisk configuration

On the Ubuntu (host) machine, in /etc/asterisk/sip.conf, I added the following entry:

[prophecy]
type=friend
username=prophecy
host=dynamic
canreinvite=yes
insecure=port,invite
qualify=yes
context=proph
auth=prophecy:none@asterisk

In /etc/asterisk/extensions.conf, I created a context proph with a dialplan that redirects all incoming calls to Prophecy:

[proph]
exten => _[A-Za-z].,1,Dial(SIP/prophecy/${EXTEN})
exten => _[A-Za-z].,n,Hangup

Configuring Prophecy

On the guest CentOS machine, in /opt/voxeo/prophecy/config/config.xml, I added the following lines in the VoIPCT category:

<category name="VoIPCT">
 ...
  <category name="Registrations">
    <category name="asterisk">
      <item name="Username">prophecy</item>
      <item name="AuthUsername">prophecy</item>
      <item name="Password">none</item>
      <item name="Domain">192.168.151.1</item>
      <item name="ContactIP">192.168.151.128:5060</item>
      <item name="ExpirationTimeout" type="int">3600</item>
      <item name="Registrar">192.168.151.1</item>
      <item name="ResolveRegistrar" type="int">0</item>
    </category>
  </category>
 ...
</category>

Here, the IP address 192.168.151.128 is the address assigned automatically by VMware to the guest, while 192.168.151.1 is the address of the host.

To call an application, I use SFLphone, an open-source softphone. One particularly appealing feature of this phone is its support for both the SIP and the IAX protocols. It is thus well suited for use with Asterisk.

Voilà! I am now able to make calls to VoiceXML applications from the comfort of my Ubuntu machine using only free/open-source solutions.