Can you not send a direct message on Twitter? I have been in contact with them several times in the last few days and they have replied within 4 or 5 hours every time. If you send a message there, at least it will be a 'timestamp' on your attempts to cancel the booking, which may help in any subsequent requests for a refund.

I think they are really trying their best to deal with all calls, but they are simply overwhelmed.