Why does my BT phone say line in use?

Regarding this, how do you clear a busy phone line?

Using Busy Line Redial

1. Hang up the handset.
2. Lift the handset and listen for a normal dial tone.
3. Press *66 or on a rotary phone, dial 1166, then hang up the phone.
4. Your phone continues to monitor the number for up to 30 minutes and a special callback ring alerts you if the line becomes free.

Subsequently, question is, why does my BT phone say check the line cord? The "check line cord" is the standard message that cordless phones display when they are not detecting a dial tone. As Keith suggests try a corded phone into the test point of your main socket. If you do not get a dialling tone you will need to report the issue to your comms provider.

Similarly, why is phone line not working?

Unplug all phones and equipment from all telephone sockets. If you have an old style phone that can't be unplugged then you'll need to get in touch. Plug a working phone (not cordless) into the main phone socket (this is usually where the phone line enters your premises). See if you can make calls from it.

How do you reset a landline phone?

Steps

1. Unplug the phone that doesn't work. Physically unplug the phone and the cord from the wall.
2. Find a phone in the house that works.
3. Unplug the working phone and cord.
4. Plug in the phone that wasn't working.
5. Check for a dial tone.
6. Try a different phone cord.
7. Consider repairing the wall jack.