Specifying a SendFAX recipient

There are plenty of example of the use of res_fax_digium and SendFax, however, none of them show how to specify a target telephone number to fax to.

For example, one might have a special outbound fax context that is only ever routed to by third parties apps via AMI actions. In the Action, the fax context and channel variables can be set (among them a recipient), but how do you pass this phone number to SendFax?