The "Line in Use" message often occurs when there is no dial tone. You'll need to identify where the dial tone problem is coming from. First, check that the base has power and try a new RJ-11 cable between the jack and the base. I like to use a old corded handset phone for additional testing. First plug in a different phone to the jack. Do you have a dial tone? If you do, then the CS6482 has failed.
If there is no dial tone, next try plugging the test phone into a different phone jack, if one is available. If there is a dial tone at another jack and a new cord didn't help at the first jack, then you have a wiring fault at the first jack. This could be in the jack itself or in the wiring to where the phone service comes in. You can fix the jack more easily. Remove the cover plate and determine the type of jack and look for a loose wire. If the wires are attached to screws, you remove the insulation from the end of the conductor and slide it under the appropriate screw. Then tighten the screw onto the wire. For keystone jacks, you'll need a push-down tool to press the wire back into place. The tool is sold in most hardware stores in the electrical aisle. Do not strip the wire before pushing it into place. See http://cablesupply.com/content/47-telephone-connections-and-rj11
to match the wire colors to the correct pin on the jack.
If you do not have a dial tone at another jack or do not have another phone jack in the house to test, find the service box. Open the box and check the connection type. If you find an RJ-11 jack, use your test phone to check for a dial tone. If there is no dial tone or it isn't an RJ-11 jack, contact your telephone company. They can test the phone line and will repair the wires for free if there is outside damage. If the problem is inside the walls, the telephone company will charge a fee to repair the issue. (This may be easier than finding a broken wire in the wall yourself.) Note: some telephone companies are trying to move residential units to newer fiber optic services and do not want to repair the copper wire to the door. This can cause problems with some security phone alerts.
I know at least twice we lost the dial tone on copper wiring after rodents chewed on the wires. That issue hasn't repeated since we were upgraded to fiber. However, during the upgrade, the installer dropped one of the RJ-11 lines behind the siding on the outside wall. Thus one phone was without a dial tone, we found the dangling wire and restored the phone service in seconds. We also lost the dial tone after contractors dug into the underground telephone service up the road from us. That was repaired quickly.
I hope this helps.