Tel & Internet dual connector-Not working-Help

[Blair]

My dual connector which connects my telephone and cable for the modem does
not allow either to work if both are connected.
If I disconnect the modem the phone works
If I disconnect the phone the modem works
I did have a problem with my extension to another phone but I am now
directly connected to the socket which does not include the extension
Can anyone suggest what is wrong and what is the solution please
Regards
Blair Malcolm

 

[Blair]

My dual connector which connects my telephone and cable for the modem does
not allow either to work if both are connected.
If I disconnect the modem the phone works
If I disconnect the phone the modem works
I did have a problem with my extension to another phone but I am now
directly connected to the socket which does not include the extension
Can anyone suggest what is wrong and what is the solution please
Regards
Blair Malcolm

Further information
BT have checked the line and it is OK
Since my last message I have found that when both are connected I am able to
connect to the Internet after 3 or 4 goes at trying to connect.Up until then
I get a message"No dial tone"
But with both connected if I try and ring in via my mobile I get the message
"busy"
So it looks like my internet connection is causing the problem
Blair