can receive fax, but cannot send fax out.

We have a fax machine working fine before. Recently it cannot send fax out, but can receive incoming fax. I have tried with two different fax machines, and had the problem for these 2 fax machines.  Error msg for these testing is  "busy/no answer". (These three machines are 2 Xerox 3550, 1 Brother 7340).

I then connect a analog phone(no screen) to the fax line to call a phone number, I can talk. Then I called fax numbers to test. Most times I hear a busy sound, but occasionally I hear sounds like fax.

Please give me suggestions for further troubleshooting. Our fax/phone system is SX-200 ICP.