REST Communications Markup Language Overview

The REST Communications Markup Language represents a set of instructions you can use to tell CPaaS what to do when you receive an incoming call or SMS.

When someone makes a call or sends an SMS to one of your CPaaS numbers#, CPaaS will look up the URL associated with that phone number and make a request to that URL. CPaaS will read RCML instructions at that URL to determine what to do: record the call#, play a message for the caller#, prompt the caller to press digits on their keypad#, etc.

For example, the following will say a short message#, and then record the caller’s voice:

<?xml version="1.0" encoding="UTF-8"?>
<Response>
    <Say voice="woman">Please leave a message after the tone.</Say>
    <Record maxLength="20" />
</Response>

RCML is similar to HTML. Only one RCML document is rendered to the caller at once#, but many documents can be linked together to build complex interactive voice applications.

Calls from a CPaaS number to an outside number are controlled using RCML in the same manner as incoming calls. The initial URL for the call is provided as a parameter to the CPaaS REST API request you make to initiate the call.

How CPaaS Interacts with Your Application

CPaaS Request

The way CPaaS passes data to you application depends on the request method for the given URI. If the request method is GET then the data is passed in the query string or the part after the question mark. If the request method is POST then the data is sent a multi-part form data just like when a browser submits a form.

CPaaS Voice Request. Any time CPaaS makes a request to you applications it will include the following data as request parameters.

Request Parameters

Parameter Description

CallSid

The unique identifier for this call. You can use the CallSid to modify the call by POSTing to Calls/{CallSid} with a new RCML URL (LCM is not supported for P2P calls).

AccountSid

Your account id.

From

The phone number of the originator of the call.

To

The phone number of the call recipient.

CallStatus

The status of the call. The possible values are queued (for outbound calls only), initiated, ringing, in-progress, busy, completed, failed, or no-answer.

ApiVersion

The version of the CPaaS API used to handle this call.

Direction

The direction of the call. The possible values are inbound, outbound-api and outbound-dial.

CallerName

The caller ID for the caller in the case of inbound calls.

CPaaS Response

In your response to the request from CPaaS you want to provide RCML that will instruct CPaaS on how to handle the current call.

MIME Types. CPaaS supports the MIME types described in the table below.

Supported MIME Types

Parameter Description

text/xml, application/xml

CPaaS interprets the returned document as an XML instruction set.
When your application returns the RCML document the root element of the document must always be <Response> or the parser will complain.

The RCML Verbs

You can use different combinations of the verbs below to create all kinds of interactive voice and messaging applications.