1815DN fax problems

My users very often get "Communication Error" messages when sending to long distance fax machines, and I can't tell why. I have tried doing an on-hook dial to these numbers to verify they are ringing, answering, and providing a fax tone, and indeed they are, but for some reason the fax never sends even when I manually tell it to. I'm wondering if anyone has had these and fixed them, or if this is a known issue.

One thing I noticed was the answering fax machine's response tone was very short. I wouldn't think this matters, especially for an on-hook dial where I manually press Send anyway. The two machines should negotiate their connection. Some of the connections work fine but some numbers consistently not.

Anyway this fax machine is pretty weak. The QuickFax function is supposed to go to fax dialing automatically if I start dialing a number when a document is in the ADF or on the glass, but if I press a 1 at the beginning of a long distance number it thinks I am trying to make copies so by the time it realizes I'm dialing a number it has missed the first few numbers, which means the only way I can dial a long distance fax is to go into fax mode first so Quick Fax is useless.