Konika minolta bizhub 350 fax no answer

I have 4 bizhubs. All four can send faxes to some numbers successfully however, I am getting "no answer" transmissions on each machine for other numbers). If I fax using a regular fax machine to the "no answer" numbers I was gettng on the bizhub -- they go through fine. They are consistently the same numbers that are "no answer". I have had the phone lines checked and the bizhubs. I have also checked with the recipients -- they are not having problems with their fax machines and can recieve faxes from us using non bizhubs -- any suggestions??