When ever users face verification code SMS problem .They test the below cases.
Check the formate of the mobile number which is there on user profile and the format should be (Country code)(Mobile Number).
Check whether User has Changed their mobile network or upgraded IOS.
Remove the contact number from User profile and enter a different valid contact number and check if the new contact number receives a correct verification SMS with code.
if yes then the problem is with the User.
Incase if step 3 fails then raise a case to salescforce.
That's how i resolved the problem with the help of Salesforce support.