The response we send back probably shouldn't be so much of a copy of the parameters we received. From the spec it looks like we need only `TransactionId` from the original message. The other parameters we need are `TransactionTime` (we should set that to the current time it looks like), `USSDResponseString` (we set that properly already here), `USSDEncoding` (the value from the original message might be sensible but it might also be better to use `UTF-8` or similar -- I'm not entirely sure) and `action` (this should be set based on the `session_event` is the outbound message.