dun error messages&proxy server

hi folks,
working on an nt server with small business and proxy server running on it,connecting to an isdn line and firstly it was dropping the connection very intermitently,now it doesnt dial out at all,got the line checked with provider,changed modem,changed cable,error message firstly was 629,then 732,now on dial out 633,port busy?but its not,is there something in proxy im misssing ,i also re-created the dial up connection,any help here would be great,it has srv pack 6a on it aswell…?
thanks alot

gary