Funny, b/c I use exact opposite settings,
echo cancel on, jitter buffer Low, up and down etc.
The jitter buffer setting shouldn't effect fax sending, as it only effects the receive audio. The sip proxy that sends back the audio from the pstn will have a jitter buffer that you have no control of.
I actually have very low jitter on my connection, so the buffers at either end probably never grow. Any audio gap or glitch big enough to interrupts the data flow, ECM mode will often recover.
The echo cancelers at the opposite end on a pots line if your faxing to a pots line, again you would have no control of the pots echo cancelers. I tend to leave my local echo cancelers on for faxing as it is half duplex and each end uses the same frequencies. I have found sometimes bits of audio echoing back in between the page data, or when they are syncing at the low bit rate can cause it to fail reading that low bitrate data.