First thing is try changing the Detection Timespan from 0 to 8 seconds. The default should be 8, so not sure why yours it set to 0.
If that doesn’t work, then you might want to disable Modem Passthrough and manually set up the port to receive a fax.
Create a G.711ulaw only codec list and assign it to the interface and then make sure VAD and Echo Cancellation are turned off.
That should take care of it. Post back was fixed it.
More info on those settings;
When Modem Passthrough is enabled and an existing call detects a modem or fax tone, the
unit will automatically renegotiate with the far end to be modem-compatible (switch to
G.711 codec automatically, echo cancellation and VAD are disabled). When connecting
analog port to a fire alarm system, DO NOT use modem passthrough. Manually assign a
G711 only codec, then disable VAD and echo cancellation.
Specifies the fax and/or modem detection time length value. Recommend starting with 8
seconds. Range is 0-8 Seconds
Those settings were a carry over from the original pre-R10.5.3 config. After adjusting those settings and rebooting the fax machine, everything appears to be working normally. Thanks