Merge pull request #254 from agent515/twilio-call-error-31600

Return CallEvent.declined on Twilio Call Error 31600 (Busy Everywhere).

Author: cybex-dev

lib/_internal/method_channel/twilio_voice_method_channel.dart

@@ -331,12 +331,15 @@ class MethodChannelTwilioVoice extends TwilioVoicePlatform {
         printDebug(tokens[1]);
       }
 
+      // https://www.twilio.com/docs/api/errors/31600
+      // Busy Everywhere. All possible destinations are busy.
+      //
       // source: https://www.twilio.com/docs/api/errors/31603
       // The callee does not wish to participate in the call.
       //
       // https://www.twilio.com/docs/api/errors/31486
       // The callee is busy.
-      if (tokens[1].contains("31603") || tokens[1].contains("31486")) {
+      if (tokens[1].contains("31600") || tokens[1].contains("31603") || tokens[1].contains("31486")) {
         call.activeCall = null;
         return CallEvent.declined;
       } else if (tokens.toString().toLowerCase().contains("call rejected")) {