In case the user denies the needed permission in the above step, the means of verification would fallback to SMS based OTP instead of missed call. In that case, you can chose to implement your own fallback OTP infrastructure or opt for Truecaller's fallback SMS based OTP infrastructure.
If you have opted for your own SMS infrastructure, you can chose to proceed as per your own flow.
In case you have opted for Truecaller's SMS based OTP infrastructure, when you request for a verification for the user's number, Truecaller sends an SMS to the user containing the OTP. Once the OTP is keyed in by the user or automatically read ( using the SDKs SMS retriever functionality ), you need to pass the user's first name and last name along with the OTP to the Truecaller SDK method to complete the verification flow.