As far as I know, the speed of your connection is the indicator of the quality of your phone lines. There is no specific rating system for phone line quailty.
There are two init strings that I have found work for just about every modem. They are:
at&f&c1&d2 (for all modem brands)
or
atn0s37=14 (for HSP or Aztec modems)
Generally, if neither of those strings work, an init string is not going to solve your connection problems. You'd be better off updating your drivers, or getting a better quality modem. Although, external US Robotics, is one of the higher end modems, by my experience.
I recommend you check out the website www.modemsite.com. It is one of the best resources I've found on this subject.