This sets how long the backward signal stays asserted while waiting for the forward digit signal to be turned off, in compelled signaling, starting once the backward response is sent. This is the far end's half of the same compelled-signaling handshake governed by the forward tone timers on the near end, so the two sides' timer settings need to be compatible or the compelled digit-exchange handshake can stall or desynchronize on that specific trunk. A backward-tone-on timer set to 1.5 seconds gives the far end that much time to see the forward digit switch off before the backward exchange is considered to have failed for that digit.