my sister had same trouble when you phoned it rang once if she didn't answer it in time it sounded like rushing water, talktalk said it wasn't the line, she had all the extensions renewed, still the same it went on for weeks got her own engineer to check it, it was on the line, talktalk had to pay all her costs and loss of use.
So if you've removed all extensions and plugged a working phone into the incoming socket, its down to them, threaten them with ofcom if thats the telephone ombudsman.