How to use AT command to detect phone line is busy

问题

I have a USB modem. Is it possible to detect the phone line is busy or not?

I thought I could use AT Command to detect dial tone, but I'm not sure are there any commands support that.

Thanks.

回答1:

I believe you have to do this the same as a human would... take it off hook and wait for dial tone.

Ripped from http://michaelgellis.tripod.com/modem.html

D alone will take the modem off-hook and wait for a dial tone. (see X command for exceptions). The length of time to wait for a dial tone before dialing is programmable in register S6.

回答2:

If you try call, and it's busy, modem send you message BUSY or NO ANSWER

来源：https://stackoverflow.com/questions/9889505/how-to-use-at-command-to-detect-phone-line-is-busy