To further customize or integrate with Flex PBX, you must use Voicent IVR Studio, which is included in the Enterprise Editon. For more details, please see Voicent IVR Application Design Tool
If you need more control of the call, Voicent also provides developers with a gateway level interface.
Voicent Gateway is a platform for developing state-of-the-art interactive programs. The gateway supports the W3C VoiceXML standard for interactive voice/phone applications. It is also a 2-way SMS gateway for interactive text messaging application.
Voicent Gateway uses the standard HTTP web interface, which allows you to use almost any programming languge to interact with the gateway. We have sample code in Java, C#, C++, PHP, Perl, Python and VB. For more details, please see Voicent Voice, SMS, Email Developer Resource Center.
It's absolutely free to download and try Voicent's Flex PBX, IVR Studio, and Voicent Gateway software. You don't need a credit card or even to supply an email address to try it.