Add optional twilio-call-format config option

To be able to set custom TwiML send to the Call API.

server/server_twilio.go

@@ -14,7 +14,7 @@ import (
 )
 
 const (
-	twilioCallFormat = `
+	defaultTwilioCallFormat = `
 <Response>
 	<Pause length="1"/>
 	<Say loop="3">
@@ -65,6 +65,10 @@ func (s *Server) callPhone(v *visitor, r *http.Request, m *message, to string) {
 	if u != nil {
 		sender = u.Name
 	}
+	twilioCallFormat := defaultTwilioCallFormat
+	if len(s.config.TwilioCallFormat) > 0 {
+		twilioCallFormat = s.config.TwilioCallFormat
+	}
 	body := fmt.Sprintf(twilioCallFormat, xmlEscapeText(m.Topic), xmlEscapeText(m.Message), xmlEscapeText(sender))
 	data := url.Values{}
 	data.Set("From", s.config.TwilioPhoneNumber)