A bit pesky there Clive. Could be other downloads taking place while you're testing.
Frank
There's a test number 17070 that allows ringback test and quiet line. Try quiet line (option 2) when the phone works and if it's anything but quiet there's likely a connection problem on the line. If your phone allows, put it on mute as well. Do this from the master socket by removing the bottom half of the faceplate and plugging into the jack that is in behind it. That way you isolate your internal wiring and are connected directly to BT's outside line. If the problem persists there it is outside and is BT's problem.
Also worth bearing in mind these days that iPads and the like automatically backup to 'the cloud' and whilst doing so can eat up serious chunks of the bandwidth to the point that your normal usage is adversely affected.