A method of operating a control module for determining the stability of a telephone line in a network, said method comprising: i) determining by the control module an expected value for each of a plurality of parameters associated with a test telephone line, wherein the expected value is based on previously measured values of the respective parameter for said test telephone line, and the parameters comprise physical properties of the test telephone line; ii) for each of the plurality of parameters, determining by the control module a deviation value based on the variation of previously measured values for said parameter across a plurality of telephone lines; iii) determining by the control module a weighting factor for each of the parameters that varies according to the present or expected value of that parameter when compared to a predetermined range; iv) generating by the control module individual parameter performance indicators for each of the plurality of parameters associated with the test telephone line, wherein the individual parameter performance indicator for a given parameter is given by - the absolute value of the difference between the present value of that parameter and the expected value for that parameter for the test telephone line, normalised by the deviation value for that parameter, and multiplied by the weighting factor for that parameter; v) generating by the control module a performance indicator for the test telephone line based on the sum of the individual parameter performance indicators for each of the plurality of parameters of the test telephone line. i) determining by the control module an expected value for each of a plurality of parameters associated with a test telephone line, wherein the expected value is based on previously measured values of the respective parameter for said test telephone line, and the parameters comprise physical properties of the test telephone line; ii) for each of the plurality of parameters, determining by the control module a deviation value based on the variation of previously measured values for said parameter across a plurality of telephone lines; iii) determining by the control module a weighting factor for each of the parameters that varies according to the present or expected value of that parameter when compared to a predetermined range; iv) generating by the control module individual parameter performance indicators for each of the plurality of parameters associated with the test telephone line, wherein the individual parameter performance indicator for a given parameter is given by - the absolute value of the difference between the present value of that parameter and the expected value for that parameter for the test telephone line, normalised by the deviation value for that parameter, and multiplied by the weighting factor for that parameter; - the absolute value of the difference between the present value of that parameter and the expected value for that parameter for the test telephone line, normalised by the deviation value for that parameter, and multiplied by the weighting factor for that parameter; v) generating by the control module a performance indicator for the test telephone line based on the sum of the individual parameter performance indicators for each of the plurality of parameters of the test telephone line.