Flow variables is a powerful feature that allows you to create complex logic in your flow. Variables can be either created directly using the Create variable block, created with data coming from an external API using the HTTP request block or created with DTMF data requested from the other party using the DTMF input block.

These variables can be used almost anywhere in your flow, for example inside TTS prompts, inside an HTTP request body or used in a Conditional split.

Please note that variables created by blocks such as the Create variable block, will only become available after the block has been triggered during your flow. Using a variable that has not yet been generated will result in Execution errors.

Providing variables

When you initiate an outbound FlowBuilder call using the Voice API, you can supply a custom list of variables that will be available from the start of your flow. For more information, please see the Outbound calls article.

Default FlowBuilder variables

The FlowBuilder provides some default variables that contain basic information about the ongoing call.

Variable name	Description	Example value
caller_number	This is the phone number of the party that initiated the phone call. For inbound FlowBuilder calls this is the end user, for outbound calls this is the phone number provided via the Voice API. Please note that this value can also be `anonymous` if the caller is calling anonymously.	`+31612345678`
callee_number	This is the phone number of the party that receives the phone call. For inbound FlowBuilder calls this is the phone number configured to your flow, for outbound calls this is the end user.	`+31612345678`
call_id	This is a unique identifier (UUID) generated for this specific call. This identifier can be used to trace this specific call in technical tooling, such as the Voice Log feature in the Voice Insights app .	`00000000-0000-0000-0000-000000000000`
call_direction	This indicates the direction of the phone call from the FlowBuilder perspective. Will be `inbound` when an end user calls the phone number configured to your flow, or `outbound` when the call is initiated via the Voice API.	`inbound`
utc_timestamp	The UTC timestamp on which the call was started in ISO-8601 format.	`2024-01-01T16:00:00.000000Z`
unix_timestamp	The UTC timestamp on which the call was started in UNIX format.	`1704124800`