The busy signal message only indicates that the phone doesn't get a dial tone signal. This could result from a problem with the wires inside or outside the home. With some modern fiber optic systems, the modem that translates the signal back to POTS (plain old telephone system) analog could be faulty.
I like to use an old corded basic phone for most diagnostics. First, try a new RJ-11 cord between the phone base and the jack. (I've seen someone use the coiled handset cord between the phone and the jack. That doesn't work.) If the new cord works, great. Otherwise, test with another phone. If the test phone has a dial tone, then the problem is with the BT system. For cordless phones, double check if the unit is getting power. Transformer bricks should get warm. If it is under warranty, contact the company.
If you still don't have a dial tone, test the phone line at another jack for that number, if one is available. If other jacks work, then you have a problem with that jack or the wiring back to the incoming service. Most phone jack faceplates either secure the wires with screws or a keystone. For the first, you will need to strip the wires before securing them to the appropriate point. For the second type, you don't strip the wires. Instead you need a punch-down tool (available at most hardware stores in the low-voltage wiring section). Use https://www.lanshack.com/wire_phone_jack.aspx
for the correct wire placement. If you recently had someone work on the service connection, check if all of the extension wires were connected to the correct part of the service box.
If the problem isn't specific to one jack, check your incoming service box. In some cases, you can just plug in the basic phone's RJ-11 cable and listen for a dial tone. If there is no dial tone, contact your telephone company. Somewhere upstream, there is a damaged wire. For underground copper telephone wiring, the problem could be from animals chewing on the wires or someone damaging the wires while digging. Most companies should fix outside problems for free. Some will try to sell you a service upgrade instead.
If you can't test the service at the box, contact the telephone company. They have the equipment to test the line. These technicians can also fix inside wiring problems for a fee.
I hope this helps.
(Yes, I have dealt with these problems before. The service upgrade was the strangest one. They unplugged four extension wires at the start of the job. Then they lost one wire behind the siding when they finished. It took three minutes for me to find the wire and plug it back into the service box with RJ-11 jacks.)