error 691 - what is error 691 ? - get during dialup to isp

[error 691]

Error 691? What is it ?

I am a computer store owner and I am getting error 691. I can sell hardware but not troubleshoot , so wanted to know why I am getter error 691 and what can I do about the error 691.

Is the error happening because of me or becuase of my dialup internet service provider. I never used to get error 691 but now its happening so I am wondering what is the reason to my error 691. And the weird thing now is that sometime I dont get error 691 one and some time i do.

thanks for your help.