Analog Loop signalling question

I have run into a situation where several of a subs lines are not able tone dial on a line and break the dial-tone. Digits are sent but dial-tone continues uninterupted. All lines are connected to a PBX. 5 of the lines can dial out but 3 lines cannot. When a butt-set is placed on the line from the MPOE none of the 8 lines can dial out with either tone or pulse dialing. The question that I have is, what is the relevant measurements to determine why the signalling is not being picked up by the central office end. Is this a voltage issue with the CO equipment, a sensitivity issue on the CO equipment end, or an issue with the signal level that is being sent from the CPE? I found it strange that the PBX would be able to break the dial-tone but a butt set on the same line would not, this would seem to indicate that a "more hot" signal from the CPE might be the determining factor. Local loops are about 17Kft long. Equipment on the CO end can be set with a RCV gain of 0db to -9db, which would indicate a higher level of sensitivity to tone digit signalling? Thanks in advance for you thoughts and suggestions.