[Implemented] Give us an error message when a call can't go through

[eric]

Currently, when a call can't go through, the local access gateway doesn't answer the call and the RF Dialer doesn't display an error message.

The local access gateway should:

1. answer the call and play an error message (not so great, because it uses minutes);
2. don't answer and have the RF dialer display an error message (it doesn't use minutes);
3. both.

[eric]

The RF Dialer now answers the phone and plays an error message.