Why can't ExtraFax send fax using a US Robotics fax modem?

Link:  http://www.extracomm.com/Extracomm/FAQ.NSF/FAQs/ee5c3c56d62e179a482565450012895c
Apply to:  ExtraFax for Exchange; ExtraFax for Domino All versions

Last Modified Date:  07/24/2013

US Robotics may be the best selling modem on the world but their fax support is notorious. Extracomm has spent hundreds of hours to study their behaviour and recode at least three times to work around it as the chances that our evaluator is using a USR modem is very high.

A general comment is that you should set the modem to Class 1 not Class 2.0 as the manual may make you think so. USR's
Class 2.0 is just too buggy to be consider useful. Setting to Class 1 should make the failure rate for sending fax less than 5%.