The explanation would go something like this;
"When you see NO SERVICE in (
But where is an appropriate location?
1. The Notes tab. One of the very few legitimate uses of the Notes Tab. Simply add a note of NO SERVICE and be on your way.
2. Use a field. The Notes Tab may not be visible enough to satisfy some users. Why not put this value right on the contact record? So, mark their Customer Type as NO SERVICE, or something like that. This ensures the user will probably see it as they are already looking at the top half of the Contact Record.
3. Use Record Alerts. An ultra-mega answer to this age old question. You define these via Edit | Record Properties | Alerts Tab. Once an alert is enabled for a record, it requires the end user to actually acknowledge the Alert.
Pro Tip : Use option #2.