Q:

Phone app with nexmo voice app integration

I am mobile app developer. I have a Phone app with nexmo sdk in-app voice integration.
Whats the procedure to work on it.

A:

In this scenario, from will be your phone number (from which you're doing call forwarding), and not your customer 123*** phone number https://developer.nexmo.com/voice/voice-api/guides/call-flow#answer-url-payload
I suggest you have the customer call the Nexmo virtual number instead, do your validation and then connect or transfer the call to your acctual phone number via the Nexmo NCCO.
Here's the documentation for transfering a call, if your validation takes longer and you need to put the person calling on hold in the meantime: https://developer.nexmo.com/voice/voice-api/code-snippets/transfer-a-call 
If your validation is faster, here's how to connect an inbound call to a different number: https://developer.nexmo.com/voice/voice-api/code-snippets/connect-an-inbound-call

