Check for a dial tone by picking up your telephone.
If you have no dial tone, follow this article here.
Does this issue only happen when you call a specific number?
Test this by calling the same number using another telephone, or a different number entirely.
Make sure your telephone volume is not off, low or muted.
If you're using a cordless telephone, check that the batteries are charged.
Also check your telephone's connection to it's base station. Ensure the base station is connected correctly to the telephone line.
Run a quiet line test by dialing 17070 on your telephone.
After hearing the your telephone number read out, press option 2 to run the test. You'll hear "Quiet line test" spoken on repeat. Listen out for noise between the prompts and hang up when you feel you understand if noise is or isn't there.
If you're using a cordless telephone. Try another handset (preferably wired) and see if this works.
If you haven't already, test your telephone from the Master Test Socket.
You can find a video on how to do this here.
If the above steps do not resolve the problem, please contact our Technical Support team for further assistance.