[asterisk-users] How to detect a PSTN telephone is busy or not?

[Zhang Shukun]

hi,all

one problem confuse me these days. i want to sequence dial three PSTN
number(a,b,c)

first, if i dial number a, if a is busy , i will dial number b. if b
is busy, i will dial number c.

Dial(SIP/a at IP,30)
Dial(SIP/b at IP,30)
Dial(SIP/c at IP,30)

i want to know before i dial number a, how to know if a is busy now?

if a is busy now. i will not dial a, instead, i will dial number b directly.

to summary is : in asterisk, how to detect a pstn telephone number is
busy or not before dialing it?

Thanks!

-- 
Best regards,
Sucan