Getting one way audio cutouts over an IAX connection, here is what seems to be the corresponding output in logs:
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Sep 28 10:30:05 DEBUG[19555]: chan_sip.c:1347 __sip_ack: Stopping retransmission on '691a8b7f124428f541c1cd6206721647@66.199.130.41' of Request 102: Match Found
Sep 28 10:30:18 DEBUG[19555]: channel.c:3009 ast_generic_bridge: Got a FRAME_CONTROL (3) frame on channel IAX2/voxter-jrc-out-1
Sep 28 10:30:18 DEBUG[19555]: channel.c:3264 ast_channel_bridge: Bridge stops bridging channels Zap/2-1 and IAX2/voxter-jrc-out-1
Sep 28 10:30:18 DEBUG[19555]: chan_zap.c:4750 zt_indicate: Requested indication 3 on channel Zap/2-1Sep 28 10:30:25 DEBUG[19555]: channel.c:3009 ast_generic_bridge: Got a FRAME_CONTROL (-1) frame on channel IAX2/voxter-jrc-out-1
Sep 28 10:30:25 DEBUG[19555]: channel.c:3264 ast_channel_bridge: Bridge stops bridging channels Zap/2-1 and IAX2/voxter-jrc-out-1
Sep 28 10:30:25 DEBUG[19555]: chan_zap.c:4750 zt_indicate: Requested indication -1 on channel Zap/2-1
Sep 28 10:31:04 DEBUG[19555]: chan_iax2.c:7044 socket_read: Peer lastms 1, historicms 1, maxms 2000
Sep 28 10:31:04 DEBUG[19555]: chan_iax2.c:7044 socket_read: Peer lastms 28, historicms 28, maxms 2000
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FRAME_CONTROL type 3 in include/asterisk/frame.h reports frame type of VIDEO
FRAME_CONTROL type -1 is of course not listed.
Not sure what would cause these frames to come across, but the audio cutout appears to be happening of course, because of the 'Bridge stops bridging channels' action.