Just to share what little I've found:
"Radio links are NOT telephone links. Their characteristics are
definitely different. I never heard of echo canceler on HAM packet link;
also I wonder about behaviour of landline FAX modem chips on real VHF/UHF
links, with a lot of phase distorsion because of reflections.
I think noise characteristics are also different.
Smetimes I wonder how our modems work, when I look what is comming out
from RX output. I would love to hear on any comparations between
1200 bd FSK and 9600 bits/sec Yamaha chip on average radio links with
reflections etc."
Not that I have any idea what all that means