An app I find really useful is Automated Away Text Messaging. You are able to customize text messages to automatically send to those who call and text you during the times you know you are going to unavailable (driving, at work, in a meeting, etc). You can customize who it gets sent to (your phone contacts only or anyone who calls or texts while you are unavailable). I find it very useful especially during driving when I absolutely refuse to respond to texts. You can also customize a specific message for a specific person. When you make it active there is an icon that shows up in your notification bar to remind you that the service is on. You do need to remember to shut it off when you do not need it anymore. I have it set to send my message to missed texts only, not phone calls but you can set it to both. It is 99 cents. Great support from the developer as well!