Welcome to our community and thanks for posting. Sorry to hear you're having issues with our landline, we appreciate the inconvenience caused when a noise appears on your line.
You advised you have already purchased new equipment to test, so this has eliminated that your phone equipment is not causing the issue. Have you unplugged all extension sockets and just tried the handset in the master socket?
Please can you let us know so we can assist your further?
Can we also please confirm if this is calling or receiving calls or both? Also, does this occur when you're calling different numbers or one kind of number (mobile, landline etc)? Finally, is this constant noise or does this fluctuate?