Usually, uncheck "wait for dial tone before dialing".

Unplug phone line, let modem dial first.

When you hear the "click" sound from modem, plug phone line after about 3 seconds.

Then normal dialing tone can be heard, and good internet connection can be established.

Hint: the timing after "click" sound should be tried for a few times to let you get used to.

This manual internet connection is troublesome, anyway, my inspiron can access internet again.

It seems the DELL support can not help me after I reported this problem.