twilioclient/capabilities.go
func (c *Capability) AllowClientOutgoing(appSid string, appParams map[string]string) {
	c.shouldBuildOutgoingScope = true
	c.appSid = appSid
	c.outgoingParams = appParams
}
func (c *Capability) AllowClientOutgoing(appSid string, appParams map[string]string) {
	c.shouldBuildOutgoingScope = true
	c.appSid = appSid
	c.outgoingParams = appParams
}
// Allows this client to call your application with id `appSid` (See https://www.twilio.com/user/account/apps).
// When the call connects, Twilio will call your voiceUrl REST endpoint.
// The `appParams` argument will get passed through to your voiceUrl REST endpoint as GET or POST parameters.