A method of detecting a set up signal having a predetermined frequency and used for data transmissions over a communication network comprises comparing an energy level of a filtered received signal with a first predetermined value and providing a first detect signal, comparing an energy level of a component of the received signal at a predetermined frequency with a second predetermined value and providing a second detect signal. In addition, an autocorrelation function is performed on the received signal to discriminate between the set up signal and other signals in the received signal and a check signal is provided when the autocorrelation function identifies the set up signal. The set up signal in the received signal is detected in response to the first and the second detect signals and the check signal. A method of detecting phase reversals in the set up signal is also disclosed.