>
> Using an external modem, I need to do the following things:
>
> 1. dial a number
> 2. detect if the call is answered
> 3. send DTMF tones to the remote phone
>
> I can do 1. with the command ATDT...
> I can do 2. detecting when the ringback signal stops
>
> I don't know how to do 3.

Sending DTMF is just like dialing, you can use an ATDT. Instead of doing 2)
though you can simply put a bunch of commas between the dial and the
DTMF tones you want to send so:

ATDT8005551212,,,,,,,,,,,,,1234567

Should accompish the task. This is what I used to do to automatically send
pager messages. Worked like a champ.

BAJ