There are a few reasons to get a line in use message; the message indicates that the phone does not detect a dial tone. For a simple corded phone, the problem could be the phone wire, the jack, the wiring between the jack and the incoming service or the service line. For all phone types, I use a simple corded phone for some of the testing. Try a new RJ-11 cord between the jack and the phone. Do you hear a dial tone? Make sure that you didn't use a handset cord in the place of the RJ-11 cable. Try a different phone. If there is a dial tone, replace the first phone. If you have another jack on the same phone line, try using the test phone at that jack. If you have a dial tone at the second jack, you have a wiring problem at the first jack or in the wiring to that jack. If you do not have a dial tone, find your incoming phone service box. If you are lucky, you have RJ-11 jacks inside the box. Plug your test phone into that jack and check for a dial tone. If you have a dial tone, you have a wiring problem. If there is no dial tone, the problem is the outside line. If you have a DSL line, check the DSL filter. If it's inside, you may need to replace it. If it's in the service box, ask your phone company to check it.
If you have a cordless phone system, you can have additional problems beyond those for a corded system. Check if the base has power. (Is the power transformer warm or cold?) If that is plugged in and cold, check if there is a switch or circuit breaker that is off. Replace the transformer if the power was good and check the phone again. If the transformer was warm, then check if the handsets were too far from the base. Move into the room with the base and see if the handset and base can communicate. You may want to check the find handset option. If the handset doesn't respond, check if the handset is registered with the base. You may need to replace the phone system. (If it is under warranty, contact Panasonic.)
If you have a problem with a specific jack, you can remove the face plate and check for loose wires. Some jacks use a keystone. These wires will need to be pushed back into the keystone with a push-down tool that is available at most hardware stores in the electrical section. The other type of jack uses screws to secure the wires. You need to strip the conductors and secure them in the correct position. See https://www.lanshack.com/wire_phone_jack.aspx
for the wiring diagram. (If you have the wires connected to the wrong pins, your phone won't detect the dial tone or the call signals.) If the problem is inside the walls, you may want to have the wiring done by someone else. There will be a service charge but it's often easier than accessing all of the wiring yourself. (If you have a single story home with an open basement to inspect the wiring, then it could be easy.) If the problem is at the service box and outside, then your phone company should fix the problem for free. (Some companies don't won't to do this and may try to sell you a service upgrade if you have copper wiring.)
I've lost dial tone to all of the wired phones from: a contractor who cut a wire while digging and rodents who chewed on the underground wires. A lightning strike took out the DSL filter once and killed the voice line. Since upgrading to fiber optics, I still lost the dial tone intermittently due to a faulty modem in the service box. I've lost a single phone extensions from someone using a handset cord (simple fix) and a service upgrade where the technician didn't connect all of the phone cables to the service box at the end of the job. Since the wire was loose behind the vinyl siding, it was easy to fix.
I hope this helps.