Fax reception errors

I recently put in a server with Windows 2000 SBE. I am also using it as my fax server. Sometimes it will not allow outside fax machines to send a fax in. I will come up with a fatal error on the fax. The sender just gets an error. The majority of faxes come in without any problems. It has a US Robitics modem that supports class 1 and class 2. I would appreciate any help I can get in alleviating this problem.