Firebase Phone Authentication - Long Delay & Multiple OTPs

I'm working on an iOS app project that involves Firebase's phone authentication. I have it working fine on simulator, my iPhone, and my iPad. However, now that I am in the TestFlight stage , my external testers are experiencing long delays in receiving their OTPs as well as receiving duplicates when they reach the ViewController where they enter the OTP code (This is probably due to them hitting the button multiple times).

I also have APNs enabled and working properly.

I don't have much code to share as I followed Firebase's documentation.

What could be some reasons for a long delay in receiving the OTP code from Firebase? I will be including an activity spinner in the project when users tap the sign-in button. However, I also don't want it to be spinning for a minute as users wait for their OTP.

@objc func phoneSignIn() {
    guard let phoneNumber = startVerificationView.phoneNumberTextField.text else { return }
    
    let completePhoneNumber = "+1\(phoneNumber)"

    Auth.auth().settings?.isAppVerificationDisabledForTesting = isVerificationDisabled
    PhoneAuthProvider.provider().verifyPhoneNumber(completePhoneNumber, uiDelegate: nil) { (verificationId, error) in
        if error == nil {
            guard let verifyId = verificationId else { return }
            UserDefaults.standard.set(verifyId, forKey: "verificationId")

            let vc = CheckVerificationViewController()
            vc.modalPresentationStyle = .fullScreen
            vc.completePhoneNumber = completePhoneNumber
            self.navigationController?.pushViewController(vc, animated: true)
            
        } 
    }
}

Also isVerificationDisabled is set to false.