I run into this the other day. It took a few months of The OA stating that the fax was not working.
[Fax Problem "Manual is fine... Automatic is not"
On a HP3100, there is an option to check the line for dialtone before sending a fax. Equipment is a HP3100, 3Com NBX 4.2.7 and ATC 3c10117c. 3Com's internal dialtone is more of a broken tone so when I disabled the check for dialtone. You now can send faxes automatic and not only manual.]
I don't know what other fax machines have an option to check for dialtone before sending a fax.
[Fax Problem "Manual is fine... Automatic is not"
On a HP3100, there is an option to check the line for dialtone before sending a fax. Equipment is a HP3100, 3Com NBX 4.2.7 and ATC 3c10117c. 3Com's internal dialtone is more of a broken tone so when I disabled the check for dialtone. You now can send faxes automatic and not only manual.]
I don't know what other fax machines have an option to check for dialtone before sending a fax.