Hi.
Some silly questions, not seeing what your configuration is.
First, what kind of phones are you putting the line on (14xx, 16xx, etc).
Secondly, and this has happened to me, have you verified that the change took. I have had instances where I put the change of a phone in, pressed the "save" icon, and not realized I didn't see the merge window show up. In that instance I had to close manager open it again, make my change and it took.
Thirdly, what you might wish to do is confirm you have dial tone on the line. That happened when I moved offices and the tech's gave me the wrong person's telephone line, that couldn't dial out.
Finally, open SSA, take a look the the extension, and see if the line is actually being selected.
If you want, post your config, and I can take a look at it and see if there is something blatantly obvious.
Hope this helps.