Hello,
Anyone know any coding or tutorial by using GSM for OTP ( one time password ) with keypad..
Already try to find it on google and youtube but the result is not what i expected.
Feel free to share 
i’m guessing you are hoping to use SMS.
The fundamentals aren’t all that hard to generate a random password, send SMS to the user and wait for a response within a fixed timeframe.
Where it gets trickier, is the encryption, and telling us what your return path is... (web, sms etc).
Its all doable, but needs more specification to suggest the whole solution.