use uncompressed codec, e.g. aLaw or uLaw (use the one preferred in your country; don't forget to set FAX Passthru codec as well)
turn off silence suppressor, echo suppressor
echo canceller should be enabled if V.21 protocol is used by fax and disabled if V.34 is used
set RTP Packet size to 10ms
set Network Jitter Level according expected jitter AND DISABLE automatic adjustments of it
reconfigure your fax - allow Tx and Rx speed up to 9600Bd only
Note that it may not solve your problem. Fax transmission require short end-to-end signal delay, jitter must not be so high and even low packet loss is just unacceptable at all, especially when ECM is not used. Your internet connection may not fulfill all those requirements.
In advance, SPA122 seems not to be state-of-the-art device, even with the latest firmware. It's predecessor, PAP2T offer better results on the same line.
Best advice ? Consider fax to be dead technology. Order your secretary to scan document and transmit it as part of email. Ask your provider for virtual-fax service (fax is received on provider side, then send to you as email).
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