> For the complete documentation index, see [llms.txt](https://doc.opentact.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://doc.opentact.org/how-to-build-your-opentact-application/call-flow-control/specify-callflow-for-tn.md).

# Specify CallFlow for TN

## Basic Concept

After you purchase a Phone Number from Opentact, you can associate that numbers to a Call Control APP.  Within this Call Control APP, you can define a Call Flow XML.  This Call Flow XML describes how you want Opentact to automatically answer your calls.  

You can have many TNs associated to one SIP Control APP.  All the TNs within a SIP Control APP will share the same call flow setting.

## How to define the Call Flow XML

There are two ways you can specify call flow XML.  The first way is to specify a static XML document and host it within Opentact.  The seond way is to define a call flow webhook within SIP Domain.  Opentact will send a restful api request to your callflow webhook with the caller id and destination number, and expect a Call Flow XML to be retuned in that restful interface.

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