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Some carriers may drop OTP SMS messages, especially when the message is too clear, too repetitive, or looks like a standard verification template. We tested this with Egyptian carriers and verified that this can happen. The same issue may also appear with other carriers.
Delivery is never guaranteed. Even if a message format works on one carrier, the user still needs to test and verify it with their own destination numbers and regions.

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Keep the OTP message short

For OTP SMS, shorter and less explicit wording may work better than a clear verification sentence. For example, this message may be dropped by some carriers: 
Your verification code is: 12736. This code will expire in 5 minutes.
Instead, try a shorter format like: 
login {app_name} 12345
This style may work better because it is shorter and less obviously formatted as a verification template.

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Recommendations

  • Keep the message very short.
  • Avoid long explanatory OTP sentences when possible.
  • Avoid adding unnecessary punctuation or repeated template wording.
  • Test the exact content with the carriers your users actually use.
  • Verify delivery across different countries and operators before relying on one format.

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Important note

The format login {app_name} 12345 is not a guarantee. It is only a practical pattern that may perform better in some cases, so every user should test and verify it in their own environment.