Brother MFC 3360C

I have a Brother MFC 3360C which is not listed on this site. I have a peculiar problem that Brother has not been able to help me fix. I have Cablevision for my cable TV, Internet and Telephone service. My MFC 3360C is hooked up to it for the fax line. It works fine sending and receiving faxes at all locations I have tried, except one. My insurance company can't receive faxes from me on this machine. They can send them to me, but I get a comm error when I try to send to them. I have to send my faxes to a friends house and he then sends them to my insurance company with no problems. He has Optimumvoice for phone service too. This is the second Brother machine I have had this same problem with. I returned the first which was an 885MC for the same problem. They are both replacing an older AT&T thermal paper unit that faxed to my insurance company fine, but these units just won't stay connected. After the sending page one message shows up its disconnect with the comm error. We have tried setting the machine to Basic settings and still get the same error. Does anyone have any idea what might cause this and how it can be fixed. The phone company tested the connection and said it was clean. I would hate to return this unit also because it was such a good price and if it worked would be perfect for my needs.