Dialplan functions have the following basic syntax:

FUNCTION_NAME(argument)

You reference a function’s name the same way as a variable’s name, but you reference a function’s value with the addition of a dollar sign, an opening curly brace, and a closing curly brace:

${FUNCTION_NAME(argument)}

Functions can also encapsulate other functions, like so:

${FUNCTION_NAME(${FUNCTION_NAME(argument)})}
 ^             ^ ^             ^        ^^^^
 1             2 3             4        4321

As you’ve probably already figured out, you must be very careful about making sure you have matching parentheses and braces. In the preceding example, we have labeled the opening parentheses and curly braces with numbers and their corresponding closing counterparts with the same numbers.

Functions are often used in conjunction with the Set() application to either get or set the value of a variable. As a simple example, let’s look at the LEN() function. This function calculates the string length of its argument. Let’s calculate the string length of a variable and read back the length to the caller:

exten => 123,1,Set(TEST=example)
    same => n,SayNumber(${LEN(${TEST})})

This example will first evaluate $TEST as example. The string “example” is then given to the LEN() function, which will evaluate as the length of the string, 7. Finally, 7 is passed as an argument to the SayNumber() application.

Let’s look at another simple example. If we wanted to set one of the various channel timeouts, we could use the TIMEOUT() function. The TIMEOUT() function accepts one of three arguments: absolute, digit, and response. To set the digit timeout with the TIMEOUT() function, we could use the Set() application, like so:

exten => s,1,Set(TIMEOUT(digit)=30)

Notice the lack of ${ } surrounding the function. Just as if we were assigning a value to a variable, we assign a value to a function without the use of the ${ } encapsulation.

A complete list of available functions can be found by typing core show functions at the Asterisk command-line interface.