Abstract: Apparatus to permit recognition of a G.165 sampled tone to allow for disabling an echo canceller determines a phase change in the sampled tone. The apparatus comprises a FIFO buffer which receives an input sampled tone and temporarily stores samples from the input sampled tone on a FIFO basis presenting a delayed sampled tone to an adaptive filter core. The core is also input by the input sampled tone. The adaptive filter core introduces a variable delay to the delayed sampled tone such that a phase difference between the input sampled tone and the output of the adaptive filter core iteratively achieves an integer multiple of 2.pi.. The apparatus includes a filter freezer for, after the phase difference achieves a multiple of 2.pi., freezing the adaptive filter such that the adaptive filter the delay in the signal imposed by the adaptive filter is set regardless of any phase difference thereafter arising between the input sampled tone and the output of said adaptive filter core.