If G.729 is set as prio1 codec and the sending fax machine is connected to the AP1120 it can happen, that the master call between GW and AP1120 is switched to G.711 before the DMC is established. The AP1120 detects the CNG and switches to data mode. When the receiving fax machine goes off-hook, the DMC is established with G.729. (The GW sends G.729 and the AP1120 continues with G.711 as the master connection was already switched to data mode). After detecting the CED tone on the receive side an OLC with G.711 is sent to the AP1120. The AP1120 does not response correctly to this OLC as a switch to G.711 already took place (on the master channel).The GW itself can not switch to G.711 as the acknowledgement from AP1120 is missing. Therefore the GW continues sending G.729 packets.