During call establishment, after the first message received in response to the SETUP message, on receipt of a Progress indicator information element with a Progress description No. 1 or No. 8 in the PROGRESS or ALERTING message, the public network shall switch through in the backward direction to the allocated B channel in order to enable in-band tones/information provided from the physical PINX at the destination interface to the calling user, stop timer T310, if running, and if progress description No. 1 or No. 8 was received in the PROGRESS message while T310 was running, restart timer T310.
The public network shall transfer the Progress indicator information element towards the preceding PINX.