Why does my line one keep redialing itself

We have two lines service from Frontier, just four days ago, the line 1 will keep redialing itself, but not actually dial out. And Frontier technician came today to check the problem, and found not the problem of the phone line.

When people on line #1, it will redial too, so other party can clear hear the dial sound.

We have unplug all the phone sets, and reset back to original setting, but the problem still alive.