Make call and detect DTMF signal using GSM Modem

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C#

I already have C# .NET program (that I found in codeproject.com), that can send/ receive sms through GSM modem. Now what I want is:

I need to make a call using the same GSM Modem and once the call is received, i need to make a voice command to enter the PIN (for e.g.); then we need to detect the numbers pressed by user (multiple digits). Then we need to validate the PIN and the same call will confirm the transaction.

How shall i do this. Please help. Any help will be highly appreciated.