The invention is a diagnostic tool for collaboratively testing a T1 line from remote terminals. A Diagnostic Interface Unit (DIU) containing test and monitoring equipment is connected into a T1 line close to a demarcation point. Testing from that common point, to either end of the T1 line is then performed by one or more operators at remote terminals. The operator terminals are connected via the Internet, to a common server in communication with the DIU. The operators each see the same page of a Diagnostic Web Service (DWS). Using the DWS, one of the operators causes the DIU to be switched into the T1 line and the T1 line configured and tested. All operators see the test setup and results on their own screens in real-time, making this remote testing equivalent to the operators meeting at the demarcation point and jointly testing from that point out.