I have 4 extensions in diff rooms & trying to make a call the display says line in use call cannot be made

Hello, thank you for using Howly. I’ll be happy to help you out! May I ask your name?

Nice to meet you, *****!
What is the model of your phone?

Thank you for the details!

Thank you!
Please, follow the instruction below:
1. First unplug the phone, then dial your number from a mobile phone and see if it still shows busy.
- If the line does not show busy, then there is a problem with your phone.
- If it still shows busy, make sure that the BT Home hub, and anything else that is connected to the line, is also unplugged, then try again.

When you called the line shows busy?

Did you try to unplug your phone?

Still the same issue, correct?

Please, follow the instructions below:
Go around the house and make sure that there are no cordless handsets that are left off the bases. If not, then please try this:
1. Unplug the main cordless base from the power and from the phone jack. Let that sit for about a few
minutes unplugged.
2. Then re-plug the phone base back into the phone jack and the power and let the phone reboot.
3. This should reset the all cordless phones in the home.
4. If that doesn't reset the phones, then remove the back covering of the phone and remove the batteries and reset (you will need to do that one cordless handset at a time)

Please, try it and update me of the results

Wonderful!
Is there anything else I can do for you today?

I am so pleased to see your issue got resolved!