trouble faxing to certain numbers - where's the problem?

This involves mainly 3 locations - I’m trying to make this as simple as possible:

Location A, B, C

Location A has trouble sending to location C at times (50%) (slow speeds, disconnections). It doesn’t have trouble sending to Location B but location B receives faxes via an internet fax service (efax) that doesn’t use the phone line.

Location B often has trouble sending to location A and sometimes C. Location A never connects at 9600 and usually connects 7200 or if that doesn’t work 2400 - nothing inbetween. Location B has tried 2 fax modems (seperate computers) and has tried 2 diffrent phone lines with no noticable improvment. Also has sent via efax which doesn’t always work but I don’t know the details.

Location C sometimes has trouble sending to location A and no trouble sending to Location B (efax #).

All 3 locations claim to be able to send to others with no trouble. I know that Location B gets send speeds of 14400 and 9600 normally.

Why can’t these 3 faxes play together?

I don’t know if this helps any, but I have noticed with one of our faxes (a really older toshiba machine) that it will NOT work with computer faxes at all. Seems to be some incompatability with the signal or something.