How to make Asterisk call analog PBX with extension dialing?

Question

Asterisk

There is a task: it is necessary that Asterisk be able to dial extension numbers of clients located behind IVR (analog PBX - but what's the difference?).
That is, you need to call the landline number, and then dial the extension number of the subscriber in a tone.
Please tell me how?

Answer 1

exten => _[7,8]9XXXXXXXXX,n,Dial(${Tch-Channel-1},35,mD(9w8${EXTEN:1:10})),
where:
${Tch-Channel-1} = DAHDI / g0 / 77777
77777 - number to answer in your terminology
35 - call duration on the channel
m - music in standby mode (you can replace it with r and you will hear what is happening in the channel instead of music)
D (two-tone dialing, aka DTMF. ..)
9 - prefix
w - pause 1 sec.