my friends home phone is not working.BT automated phone line checker says there is a fault between exchange and his home which there are going to fix Monday,so how come his b/band router is working fine when it use the same line from the exchange?
This can and does happen, as the broadband signal is AC, at radio frequency, and therefore able to pass over a small disconnection or high resistance joint.
The telephone relies on having a good DC path. If the broadband is working, chances are that it will be at a lower speed than normal.