"Check Tel Line" indicates that no dial tone is heard by the phone. If there are multiple extensions, check that all of the handsets are on their hooks. For cordless phones, make sure that the base has power. After that, you need to check the wiring to determine if the phone is faulty or where the wiring fault is. First, try a new RJ-11 telephone cord between the phone and the jack. (Make sure that a handset cord is not used instead.) If you have a dial tone, you have solved the issue. Next try a different telephone; I like to use an old wired phone always works with POTS (plain old telephone system) wiring. These don't need electrical power and are easy to move between jacks. If there is a dial tone on the second phone, then the first phone has a wiring fault. If it is under warranty, contact the manufacturer for repair.
Next try using the test at another jack for the extension. If there is a dial tone, then the first jack has a fault or the wiring to that jack has a problem. Open the wall plate and check for loose or damaged wires. There are two types of wiring connections - keystone and screws. For the keystone jack, you'll need a punch down tool to push the wire back into the appropriate slot. The tool is available at most hardware stores in the electrical department. Do not strip the insulation. On the other type of jack, strip the insulation and secure the wire under the screw. Use https://www.lanshack.com/wire_phone_jack.aspx
to check which wire should be secured where. Multi-line jacks must have the wires correctly paired.
If there is no dial tone at another jack or you don't have another jack for that line, find the incoming service box. If you are lucky, then the box has RJ-11 terminals. Use the test phone to check for a dial tone. If there is a dial tone, then you have a wiring problem inside. If you can't test the line or there is no dial tone, contact your telephone provider. They should repair outside problems for free; although some will try to sell an upgrade to avoid maintaining the wiring. If it is an indoor wiring problem, you may want to pay to have someone else find the damaged wire in a wall. The telephone company repair crew can do this.
Other possible causes are a damaged DSL filter. If that is inside the home, you have to replace the filter. Filters in the service box should be repaired by the telephone company staff. If you recently had a service box repair, check that all of the wires were properly replaced. My home lost one extension after a service upgrade because that wire was left loose behind the siding on the wall by the contractor. Once we found the loose wire, we plugged it in and solved the problem.
For fiber-optic systems, the problem could be with the modem. That will have to be fixed by the provider. Other problems may be able to reset at the computer at the provider's hub.
Damaged copper wires can be caused by rodents. Above ground wiring could be damaged by vehicles or weather. Underground wiring is damaged by contractors and idiots who dig in the wrong place. If water gets into the copper wiring, it can cause corrosion causing the failure eventually. (With a fiber-optic system, water damage takes incorrectly sealing the connections during the installation.)
I hope this helps.
(I've dealt with most of the above, including the loss of the fiber-optic modem. With that, we had intermittent loss of dial tone and the rest of the service. It took longer to diagnose due to the intermittent nature of the problem. We've also had multiple cases of the contractor dug too deep for both copper and fiber-optic underground wiring. A few times, the issue was area-wide since the cut wiring was near the service provider's hub.)