These calls will not ring on your phone before ringing the destination number. Once enabled, missed calls will not go to voicemail. Forward 'busy signal' calls: When you enable 'busy signal' forwarding, calls are only forwarded if you are utilizing call waiting—essentially if you're on two calls.
Besides creating an audio file in the right format (Adaptive Multi-Rate), you also need to trick the iPhone to upload your new greeting to the AT&T servers:

Here are several tips to consider when setting up a professional voicemail greeting: Keep it brief and concise. Give options to get more details. Ask for detailed messages. Be enthusiastic and identify yourself and the company name. Use positive sentences. Take messages accurately and completely. How do I change my voicemail greeting? On your Android device, open the Google Voice app . At the top left, tap Menu Settings. In the Voicemail section, tap Voicemail greeting. Next to the greeting that you would like to use, tap more Set as active. How do I delete my voicemail greeting?
Of the many things your phone can do, automatically recording a phone call is not one of them. Perhaps not including that feature is a security precaution for phone makers; it could be a potential liability issue. Regardless, from time to time, documenting a call is a useful thing to do, whether you’re conducting a phone interview or cataloging a customer service call.