Cannot send or receive faxes from one certain phone number?

Here at my job we have never had problems with our fax machine until we merged with another agency. the new agency and us cannot fax to each other for some reason. I called the phone company, and the fax machine maker - Brother. Both of them blame each other and can't understand why this is happening. Both agencies can fax and receive faxes from any other phone numbers with no problem. The fax machines are not hooked up to any computers, only to a telephone line. Each fax machine has their own fax telephone number. Whenever we need to fax each other, we have to use a neighbor's fax machine. We have been baffled by this for over a year now. I'm hoping that there is some intelligent person here on Yahoo Answers who may know why this is happening.

Ok, if I were to try to troubleshoot this problem I would unplug the fax machines and plug in a test set (or any analog phone). The call over to the distant end fax machine. If the call goes through then you know you have a fax machine problem. If the call does not go through you have a phone line problem. The problem is going to be one of those two things. Good luck, and add more details if you want. I'll try to help.

J... - Does it sound like her system is VOIP to you? I didnt get that impression.

J... - I'm not disagreeing with your input, it seems we are just starting in different places. I am looking to start at the device and you are looking to start with the network.

Bootlegteen47 - I think between the answers here you will find the solution to your fax trouble.

Your symptoms make me wonder if one of your telecom services is using VoIP as a backbone. VoIP is notorious for not being reliable when handling Fax calls.
It's usually an issue of not being able to effectively handle the tone frequencies that FAX machines require.
Some VoIP services also have problems with DTMF tones too for the same reasons.
If you test the lines with a regular DTMF telephone, be sure to not only test for dialing, but also test the DTMF tones after a call is established.
The way I test for DTMF reliability is to call a number that has menu options and you have to enter DTMF tones for your selection (i.e press 1 for A, press 2 for B, etc). I have some select weather services telephone numbers that I call to test for DTMF reliability (i.e. for local wx press 1, for marine wx press 2, etc...).
This may not solve your FAX issues, but it may indicate if the lines have any problems with accurate handling of audio frequencies besides voice.

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KMFF... it is hard to know for sure without knowing their service provider name. Too many unknown variables, really. Are the calls local or long distance?
If long distance, many carriers are using VoIP as long distance routes to keep costs down. Also, LD carrier may be different from local carrier. Hard to know really without specific details.
In the end, perhaps one of the carriers has defective hardware causing problems with the FAX signaling tones. Or, a defective cable at one of the agencies in question. In which case, you are right that they would require test equipment to evaluate the cable quality. But, doesn't sound like the carrier is taking any responsbility to follow-up...