A. SMS OTP verification
An SMS with text OTP from server is sent to the phone number registered by user through a SMS gateway, user submit the OTP to server and get their mobile number activated after successful verification. 

① User initiates mobile number verification request
② Server sends SMS request to SMS gateway
③ SMS gateway sends SMS with OTP [2407] to smart phone [0975000222]
④ User is asked to submit OTP, the mobile number is activated after successful verification

B. Inbound call voice OTP verification
An automated voice call is made to the phone number registered by user, it will play a voice OTP code. User is asked to submit voice OTP, get mobile number activated after successful verification 

① User initiates mobile number verification request
② Server sends voice OTP and user number to phone authentication system
③ System makes call to user registered number [0975000888], play the voice OTP [2407] to user
④ User is asked to submit voice OTP, the mobile number is activated after successful verification.

C. Outbound call voice OTP verification
User dials system phone number with registered mobile to obtain voice OTP. Enters the voice OTP in website, get the mobile number activated after successful verification. 

① User initiates mobile number verification request
② Server sends voice OTP and user number to phone authentication system. The system number [02-2999-6666] is provided to user at meantime
③ User calls [02-2999-6666], system gives back the voice OTP [2047] based on user’s Caller ID
④ User is asked to submit the voice OTP, the mobile number is activated after successful verification

D. Outbound call verification without answering (24x7 solution)
User calls system provided number with registered mobile number and hang off after few rings. Without answering the call, system will get the user’s Caller ID and send it to web server. The mobile number is activated after successful verification

① User initiates mobile number verification request
② Web page provides system number [02-2999-6666]
③ User dials [02-2999666] with registered number [0975000888]
④ System receives Caller ID [0975000888] and sends it to web IP promptly. Thus, the verification completed, the mobile number is activated after successful verification

E. Inbound call web OTP verification 
During verification process, web server will generate an OTP code automatically. It will be displayed on user’s webpage. The system makes call to user’s registered mobile number and requests OTP input. The system and webpage will notify the verification result promptly.    

① User initiates mobile number verification request
② OTP [2407] is automatically displayed on webpage, server sends user’s registered number to system instantly
③ System calls user’s registered number [0975000888], OTP [2407] will be entered by user
④ System returns code to web server, the mobile number is activated after successful verification

F. Outbound call web OTP verification
During verification process, web server will generate an OTP code automatically. It will be displayed on user’s webpage with system phone number. It requires user to make a call to system with registered mobile number, the OTP input is requested then. The system and webpage will notify the verification result promptly.

① User initiates mobile number verification request
② OTP [2407] and system number [02-2999666] are automatically displayed on webpage 
③ User dials system number and enters OTP [2407] by keypad
④ System returns the code to web server, the mobile number is activated after successful verification