While looking for information on a Boca modem that contained a Cirrus Logic chipset (CL_MD34xx), I found a good AT command reference manual for this chipset at:
http://www.logicode.com/download/cir33txt.exe - which is a command reference manual for a Logicode modem with the same Cirrus Logic Chipset.

After many connection failures (even with speeds as low as 14,400), a technician from Boca said that downloading:
ftp://ftp.bocaresearch.com/Drivers&Software/File_area_14/ - The file needed is called 4385dos.zip

I have not verified that this resolved the issue, but I have not yet heard back from the customer - which is usually a good sign!