Inconsistancy with Firebase Phone Authentication

My Firebase phone authentication is inconsistent with sending the verification codes. Out of 6 testers, 3 received the Firebase code while 3 of the testers did not receive the code. All 6 testers are using IOS 10.3.2. Firebase does not return an error when the phone number is submitted but the code is still never sent for half of the numbers. There is no error when verifyPhoneNumber is called which means the verification code should have been sent. Has anyone found a solution to this problem?