it will be your line.i had a similar problem and mine was eventually resolved by bt reducing background noise and tweaking something else.i contacted bt twice who twice told me there was no problem but i eventually got my isp to speak with them after they had also suggested many many things all of which were wrong !
good luck getting them to do it tho it took me months.keep on to orange who will keep advising you to try this and try that but eventually they will concede that you are right !