I'm posting on behalf of my elderly parents, so please excuse the sketchy details, they live in a different town to me.
They recently had a hub upgrade (not sure what hub it was) and an engineer came out to fit it all for them. He plugged their phone into their hub, which was fine as it was a corded phone. Since they bought a twin set of cordless phones (Panasonic) there is lots of interference on the line. There's crackling and also it's breaking up, it gets worse the longer they're on the phone.
Does anyone have any ideas why this is happening please? Speaking to them tonight it appears they have the hub and the phone base on top of their desktop computer box - could it be an issue with static or something?
For the purposes of a test, try moving the cordless base station away from the PC and the hub (within the limit of its cord) so, as far as possible, it is in a free space and not near other electrical equipment. See if that improves the interference problem. The PC or hub may be may causing interference.
The early pages of a Panasonic phone manual usually gives a lot of info about suitable placement of the base station.
Welcome to the forums and thank you for taking the time to post.
I am sorry to hear that your parents are having some issues with there landline.
I think a good place to start would be to do as goslow has advised, If they are still having issues after this then we can most defiantly take a look at the lines & see how we are able to assist further.