A "Line in Use" message generally means that there is no dial tone detected. Assuming that all of the handsets are in Standby and any other phones are connected to that line are also on their hooks, then you need to determine why the base isn't getting a dial tone. This can be an inside or outside wiring problem.
I like to use an old wired telephone for testing since power isn't required to use the phone. You also need another RJ-11 telephone wire. Try connecting a new RJ-11 wire between the KX-TGA131 base and the phone jack. If there is a dial tone with the new cable, you had a bad cable. If not, try the alternate phone. If the test phone has a dial tone, then the KX-TGA131 has failed or has a power problem. If the phone is under warranty, contact Panasonic.
If neither phone has a dial tone, take your test phone to another jack connected to the same phone line (number), if available. If you have a dial tone here, then the problem is in the first jack or the wiring to the jack. You can try to repair the jack yourself. Remove the cover plate and look at the jack. If any wires are loose, reconnect them to their appropriate location. See http://www.lanshack.com/wire_phone_jack.aspx
for the usual patterns. There are two types of telephone jacks, keystone and screw terminals. For the keystone type, you need a punch-down tool (available at many hardware stores in the electrical section) to put the wires back in place. Do not strip the individual conductors before using the punch down tool. For screw terminals, strip the wire before securing it under the screw.
If you do not have another jack or there is no dial tone at the second jack, find the incoming telephone service box. Open the box and check the wiring. If you find an RJ-11 jack, plug in your test phone and listen for a dial tone. If there is a dial tone, you have a wiring problem. Check the jacks as above. You may want to have someone else check the wiring in the walls if there are no broken/loose wires in the jacks. Your telephone company will usually offer the service for a fee.
If you can't test the phone line at the service box or there is no dial tone at the service box, contact your telephone company. They will test the service box and should repair outside issues for free. (Some providers are trying to avoid copper repairs.)
If you have DSL, you may need to check the line filter. This filter can disrupt the dial tone when it fails. If the filter is in the interior, you are responsible for the replacement. In my service area, the DSL filter was in the service box so my telephone company repaired it.
I hope this helps.
(I've had no line issues several times. Sometimes, it was the result of a contractor cutting through underground lines. With copper wire, rodents sometimes damaged those underground wires. Once it was simply an issue of a contractor who'd left one wire loose after upgrading the service.)