[asterisk-users] Detecting answer with an analogue card

[Eric "ManxPower" Wieling]

Stefano Corsi wrote:
> "Eric \"ManxPower\" Wieling" wrote:
> 
>>> There's still something I don't understand: when using a simple modem 
>>> on an analog line, you get correct answers from the modem: "NO 
>>> ANSWER", "BUSY", "NO DIALTONE", etc... why is this possible with 
>>> these TDM2400 cards that cost twenty times as much?
>>> I know that I'm probably missing something... could you help me 
>>> understand what?
>>
>> Because the far end sends a carrier tone.
> 
> Uhm... but wait: I remember old days modem to have also a VOICE return 
> code... again, why should a 20$ box be able to detect VOICE while a 
> 2000$ card shouldn't?

This stuff is not done in the card, it is done in software in Asterisk 
(or Zaptel).  Apparently nobody has cared enough to do the very complex 
audio processing and DSP work to make this work reliably .