This app watches the number that you, your contact list, or another app dials and, if it matches a user defined rule, will add / delete digits you define. I use my cell phone for business. I don't want the people I call to know my cell phone telephone number because I want them to call back at the office, not my cell. So my cell is configured to block my Caller-ID for ALL outgoing calls. Yet, I want to use apps like Slydial and SpoofCall. These apps "call home" and use the caller-id to login to the service. But my caller-ID is blocked and I can't change the number the apps call to add *82 to unblock caller ID since the "call home" number is hard-coded. So I have not been able to use these services. Enter Prefixer. It automatically intercepts the outgoing call and add *82 to the number before the call is placed. Problem solved. This app supports wild card character so, for example, if you wanted to block caller ID whenever you called a range of numbers (e.g. all 800 numbers, all phones at a certain business, etc) it would be no problem. From the documentation, it appears that this app has great use in Europe to add country codes and the like, but there are plenty of uses here in the USA as well. Rock solid programming as well -- never a force close. I've been looking for something like this for a long time.