You can record a call either by using the Voice API or the Conference XML element.
Using the API
You can record calls using the Voice API at any time during the call. The API returns a URL that specifies a file that contains the audio recording. Our documentation explains how to get the call_UUID of an ongoing conference call, which you need to record a call.
Using record attribute in conference XML
You can set the "record" attribute to "true" in the Conference XML element to record a conference call. For example:
<Conference record="true">My Room</Conference>